In advertising, customers are routinely shown to be extremely and conspicuously ecstatic about the product, no matter how mundane - or in fact boring - it is. This works in all formats: visual forms of advertisement may show people grinning inanely, laughing and jumping for joy, radio spots may have people cheering and going woo-hoo in the background because they saved pennies on a bubblegum purchase, got a free sandwich with their car or are just generally pleased with the overall product because, you know, it's just so good.
The trademark stupid grin fits well in the psychological profile of someone who is Too Incompetent to Operate a Blanket and in touch with The Power of Cheese. When taken Up to Eleven, it can give off Happiness Is Mandatory vibes or alternatively sexual overtones when the ficticious customers seem really turned on by something that's usually considered unerotic in Real Life, unless you're into that kind of thing.
Often happily married to Magically Delicious.
See also Stepford Smiler.
As this is an Omnipresent Trope in advertising, there's no need to list straight examples here. Feel free to list this trope on the pages of works where it occurs.
- Parodied in The Austin Lounge Lizards music video "The Drugs I Need."
- In Batman: The Animated Series, one of the Joker's schemes involved making a commercial. Even with the Joker's usual level of trademark enthusiasm, the commercial barely seemed out of place.