Parodied in El Goonish Shive, in which several aliens and magical beings successfully disguise themselves with shirts and hats reading things such as "Homo Sapiens" or "Ordinary Student". This filler comic serves as an example.
George the Dragon is infamous for using and abusing this particular trope, usually to the disgrace of any human beings present.
This is an example where the dragon 'sneaks' into a top secret meeting of the Dragon Hunters Anonymous.
Also parodied in 8-Bit Theater where the bad guys sneak into a castle by hiding behind a banner with "Doin' fine" on it.
Subverted at yet another point, where Black Mage kills an evil cultist and slices off his face to use as a mask. Upon greeting the other cultists they immediately realize that he killed their friend and is using his face as a mask, and lecture him on what a poor disguise it is.
Parodied in The Adventures of Dr. McNinja, where the doctor tries to achieve this using only a name tag. No one's fooled — the mask, you know — but they play along. In fact, every single time he tries to disguise himself, he leaves his mask on. Apparently Contractual Genre Blindness is not just for villains these days.
Subverted in Narbonic, where a group of intelligent hamsters operate a fake body with a paper plate with a face drawn on — poorly — for their disguise. They can't even get their pronouns right. Nobody is fooled, but tend to take in stride the fact that they're talking to a bunch of hamsters.
Another subversion in The Law of Purple: the human characters disguise themselves with facepaint to pass as Caligulians, but not only is this not convincing, none of the natives seem to care whether the humans are disguised or not. They ditch the facepaint relatively quickly.
With their robots even this would be too sneaky. This one believed when robots that disassembled him, basically, told him they're handless models. It gets better when Edge turns off his transponder, other robots don't even recognize him as a robot. Conversely, recognize as a robot anything with a proper transponder.
Robots are programmed to identify each other by transponder signal — which makes sense as they can move their "brains" around. It also makes a perfect disguise, to the point that they can have a masquerade this way. And see "ghosts". They are not stupid or gullible, they just don't have the necessary shape recognition algorithms.
In the storyline where Roomies! started getting really strange, a group of aliens escape the notice of the general populace by putting funny shapes on their heads and claiming to be Teletubbies.
In some early Sluggy Freelance strips, Aylee went out in public wearing a hat and trenchcoat to disguise her alien appearance, which surprisingly worked. In her latest form, she goes through much more effort to create her disguise.
Bun-Bun and Kiki have also operated robot versions of Torg and Riff on occasion, which people can't seem to tell apart from the real things despite their obviously blocky appearance.
Sasha dresses up as the supervillainess Monicruel in this strip. It works perfectly because, as Crushestro put it, "Boobs and a monocle. Who else could it be?"
Building 12: After The Reveal in the first chapter that Alex is a girl, she's generally drawn in a way that, while still fairly flat, she's not likely to be mistaken for a guy. Somehow, The Masquerade remains unbroken.
Batman decided to crash a 'Welcome Back' party for Hal. He really didn't try hard. Batman and Sons.
Slightly Damned pulls this off twice with Buwaro - first he wears a pimp suit which doesn't really cover his horns or fur (people seem too busy proclaiming his friends as hookers to notice), then he explicitly wears no disguise and everyone assumes it's a perfect costume of a Demon.
Subverted in "Super Temps" as most people see right through the disguises, and just go along with it anyway because the supers themselves are loopy and rather sensitive. Bonus points for the fact that many of the supers themselves not only buy into each others' paper-thin disguises but also think that the civilian populace's paper-thin facade of being fooled is real.
Played straight in Zorphbert And Fred, as none of the humans notice the intelligent behaviour, human mannerisms and bloody obvious antennae on the title characters, who are aliens disguised as pet dogs to study Earth.
In one page of "Spiff Spoonerton and the Planet of Hot Green Women" involves Spiff and Miri infiltrating a military base. Miri wears a maid uniform and does nothing to obscure her face. Spiff wears his normal clothing with a piece of paper that reads "Also Maid". Exceptional in that Spiff is literally the only human on the entire planet and both are well known outlaws.
Miri: I'd be more concerned about how well that went if I wasn't still hung up on why you had a maid uniform in a single-person space craft on an exploratory mission."
Spiff: A prepared explorer never neglects the possibility he'll need a disguise.
Miri: But why a human female housecleaner disguise?
The Princess uses this. She really should take off her hat first.
Used again by Prestige. It's her ability to fake an illithid's smell that helps pull this off.
In Captain SNES, King Hippo successfully pulls off an Abraham Lincoln disguise with a fake beard and top hat. He explains to Alex (who isn't fooled by the disguise for a moment) that it is the foolproof "Niht Repap" technique, and that Alex must be the Gamemaster if he's capable of seeing through it. Alex then bribes King Hippo with a beer into teaching him the technique so he can escape from prison.
In Goblins, Fumbles' plan to protect the warcamp from attacking adventurers by disguising everyone with fake moustaches is initially decried by Chief as 'the dumbest plan ever,' but when the only other ideas the goblins can come up with are 'hope the adventurers poke themselves to death with a pointy stick' and 'cover everyone in their own faeces', it's the plan they eventually go with.
The Buttersafe strip "The Essence of Being Human" features an enormous, disgusting Eldritch Abomination with multiple tentacles and mouths all over its body. It disguises itself using an extremely small mask. A passerby accuses it of being a monster...until the creature suggests "watching cat videos on the Internet." The passerby then immediately warns the hideous beast that "there's a monster somewhere around here pretending to be a human."