Very true, and it applies to mechanics as well - it's not what's written on your character sheet that matters, it's what results that produces in play. If you have to say "just look at my character sheet* / read my backstory and you'll see that my character's interesting" then it isn't yet interesting, and you should figure out how to convey that interesting-ness in play.
Yes, this does mean that "character who's been involved with all kinds of crazy **** but pretends to be totally normally (and is good at that)" is a rather difficult concept to make work unless you get some help from the GM (having that crazy **** come up in-game in ways that make sense). I've tried this and failed, and I realized the problem was that in media, you'd cut-away to the character's flashbacks or show them doing stuff alone that contradicted their mild-mannered image, but in most TTRPGs that kind of "camera shift" isn't a thing, and would be kinda spotlight-hogging if only one player was doing it.
Also true, but I don't think hamming it up is a bad thing. IME, in the significant majority of campaigns the characters aren't super-deep, are in fact somewhat stereotypical, and that's fine.Of course, what many players forget is that under the same principle, no one cares or even remembers that you are a Goblin or Catfolk or Edgelord instead of a human. Unless it becomes relevant at the table. And even those players that don't forget that tend to try and address it by hamming it up.
*Ok yes, there's also being interesting on a purely mechanical level, like "I made a healer+exorcist without any levels in divine classes", which does generally involve looking at the character sheet, but that's different than being interesting in-play.