Femininity Failure

Nowadays, it's considered much more acceptable for a girl to be tomboyish than it was years ago. Still, sometimes, The Ladette will give femininity a try, even if it's just for one episode. Maybe she wants to look and act elegant for a special occasion, or she's aiming to fit in with her friends, or she wants her Love Interest to see her as more than just "one of the guys". She may take it upon herself to learn ladylike behaviour, or she'll have a feminine friend teach her to act prim and proper.

Either way, she'll suck at it.

If she takes cooking lessons, she'll end up burning water. Any attempts to apply makeup will leave her looking like a clown. Walking in high heels will result in her taking a tumble. Even after she learns polite manners, you can bet she'll swear/gobble her food/belch at the worst possible moment. Old habits die hard indeed.

This can lead up to An Aesop of the Be Yourself variety, that it's okay for a woman to be masculine if she's most comfortable doing so, and she shouldn't change herself to please others. It can, of course, just as easily be full of Unfortunate Implications, either that something's wrong with her because she can't act feminine, or that her inherent tomboyishness is a sign that she's better than girly girls.

Super-Trope of Feminine Women Can Cook and Maternally Challenged.

Contrast Girliness Upgrade and She Cleans Up Nicely, for when the attempt at being feminine works.

Examples of Femininity Failure include:

Anime and Manga

Love Hina has Samurai girl Motoko forced to try becoming more feminine after Keitaro (more or less accidentally) defeats her in a duel. She's too violent to be good at it, though.
Generally the case with Akane in Ranma ½.
In Kashimashi: Girl Meets Girl, tomboyish Tomari tries to teach the recently-female Hazumu how to act like a girl. Although she does impart some wisdom, mostly all she manages to do is prove that, even when she was in a male body, Hazumu was more girly than Tomari.
In Berserk despite Guts' memorable reaction to seeing her in a dress, the femininity is somewhat dampened a moment later when she elbows him on the jaw for staring. She does this in front of several noble ladies who are very embarrassed to have seen it.

Comic Books

Lucky Luke: One story has Calamity Jane taking etiquette and cooking lessons. Needless to say, she can't help but swear like a sailor and her cookies are inedible. When threatening a group of men with her rifle unless they eat up, one of them opens his shirt and says "Fire!"

Film

The plot of Miss Congeniality.
Mulan has the title character screwing up the traditionally feminine "matchmaker" process.
Sylvia Scarlett has the title character after only a short time of dressing up as a man somehow incapable of passing as a woman (she now has short hair, and masculine mannerisms so even wearing a dress is an Epic Fail).
In Shrek, Princess Fiona struggles with her role as a stereotypical fairytale princess.
In The Untold Story, the female police officer dolls herself up to impress the guys at the office but ends up going too far and looks like a prostitute.
The Princess Diaries has the awkward Mia who takes princess lessons and gets a makeover to which her friend tells her that she sold out. She's still clumsy after the lessons.

Literature

Sarene in Elantris is a princess who tries to be traditionally feminine, but fails spectacularly at girly activities, leading to her having trouble fitting in with the court.
Hilarious example in Monstrous Regiment by Terry Pratchett, where an entire platoon of women pretending to be men have to go undercover in "drag", and fail miserably at it. For extra hilarity, the one actual male in the group has no trouble whatsoever passing as female.
In the Chronicles of the Kencyrath, heroine Jame is an Action Girl par excellence and all around harbinger of chaos being a nascent avatar of destruction will do that for you, and needless to say, when she's forced to try and become a proper Highborn lady in the third book (translation: elaborate dresses, lots of sewing, power wielded primarily through political intrigue, and very little in the way of physical activity), hilarity ensues. And by hilarity, we mean "rampant anarchy".
Several decidedly unfunny examples happen with Brienne of Tarth in A Song of Ice and Fire. She is so huge and ugly that attempts to dress her in a feminine way make her look worse than she does in armour, and while it provides some amusement for other characters, it only increases her feelings of being a freak unloved by all.

Live-Action TV

In Degrassi Junior High tomboy LD tries to be more feminine, including getting a makeover from her best friend Lucy. It fails and the makeup makes her look like a clown.
Sam from iCarly did this to get a guy's attention, with Carly giving her help to embrace her "inner daffodil". She ends up fighting with another girl by the end of the episode. As it turns out, the guy was impressed and said he didn't want to date "some daffodil."
Happens on Naturally, Sadie when Sadie gets upset after being called 'son' and tries to become a girly girl. Her mother is away so she gets advice from Hal (of all people). By the end of the episode, she has managed to find a balance between tomboy and girly girl.

Web Comics

From Gunnerkrigg Court: Kat, the more tomboyish of the protagonists, puts on makeup for the first time in order to impress Alistair, a New Transfer Student at her school. She fails horribly, and Annie's there to assure her that she's lovely just the way she is.
Falco from Namir Deiter tries to act more feminine for a while, when she's worried Cedric will go back to Tipper.

Web Original

The Nostalgia Chick tried to invoke Kawaiiko for a while, but could never pull it off. Now she's settled for being a Ladette Psychopathic Womanchild.
- Her friend Nella was given a visit by the makeover fairy. She resisted and the fairy ended up putting a bag over Nella's face because there was nothing she could do for her.

Western Animation

Kung Fu Panda: Legends of Awesomeness: Tigress, in "The Princess and The Po" tries to poses as the princess to protect the actual princess from the bad guy. She tries to do the Princess Ceremonial Dancing Song. She can't sing, and she can't do graceful ladylike. (although the real princess is not much better)
Arthur: Francine tries to be feminine for one day, picture day, where she goes to school in a dress and doesn't play kickball during recess so she'll be presentable for her photo. But she ends up playing anyway, and being scruffy in the picture.
An episode of The Legend of Tarzan has Terk wanting to be feminine to attract another gorilla. She realizes by the end that it's completely unnecessary.
The Hey Arnold! episode "Helga's Makeover". She does this to attend Rhonda's sleepover but reverts to her old self after refusing an avocado cream mask.
In The Legend of Korra, Korra decides to try out that "powdering your nose" thing she's heard about. The ensuing cloud of dust leaves her coughing.