Paper-Thin Disguise/Fan Works

The three escapees continued making progress, slowed only a few times when they were spotted by ninjas, all of whom held brief discussions of questionable logical validity before agreeing that the three couldn't possibly be who they very obviously were.

—Uninvited Guests

Examples of Paper-Thin Disguise in Fan Works include:

A large degree of Kirby fanfiction gives Kirby the ability to become completely indistinguishable from whoever he's taken the power/costume of, despite the obvious size differences.
Kyon manages to call Tsuruya and make her think he was his uncle Kintaro in Kyon: Big Damn Hero, even though he sounded identical to normal. He was sleeping in the same room as her at that point, though.
The incredibly useless Invisibility Cloak in My Immortal: people can see the cloak when it is in use, so basically the users are walking around with a regular cloak over their heads.
According to Light and Dark - The Adventures of Dark Yagami, a naked woman can pass for a man merely by having a tattoo that reads "IM A DUDE".
In the Calvin at Camp episode "The New Kid," Larry Koopa infiltrates the camp while wearing nothing but a Mickey Mouse hat.
The Hazbin Hotel inspired fan comic seen here. Katey Killjoy is at the eponymous hotel hoping to confirm rumors that Charlie is harboring a human; the person she interviews (an Author Avatar) is clearly human, his only disguise being Halloween-style fake devil horns. Katey doesn't notice, but then, as most fans can attest, she isn't very smart.