Many a Star Trek alien has bumps, ridges or markings on their face or head.
In universe, has any alien ever commented on how humans have suspiciously even skin tone, lack of markings and extra head organs, smooth faces and foreheads, relatively simple hair and ears, etc.
A comment about humans in general among the species of the galaxy would be preferable, rather than just comparing humans to their own species.
Humans don't have to be the plainest (meaning simplest features) and we don't need to keep arguing over 'a matter of perspective' for the question to make sense. (On scifi.SE.nausica, if someone wants to ask the converse question, he/she can go ahead). I never claimed humans are the extreme of the extreme or that it can be objectively defined. And it's not a requisite for the question to be meaningful. The fact is humans are simpler than a vast majority of species is adequate. (For example, the Odo counterexample.)
It also doesn't matter that you think a Cardassian and a Klingon will argue about who has more complicated foreheads. That's not germaine either.