To make them stand out from regular looking characters
You're going to need to explain to me what's off about Johnny because I'm not really seeing it. He has a relatively normal shape (in cartoon terms) and has blonde hair and blue eyes and he is edgy by spiking it and giving it red flame highlights.
there's nothing off about johnny, I didn't think the hair was died (never heard of that), that's the reason why I put him aside, because he doesn't really look bad but he has a weird trait, his hair, which as far as I know was never explained as died hair, so it seems as it is his natural hair,
I am not by any means pissed by a giant head, but it was constantly being mention by his female pairs and was being brought up all the time (Talking about Jimmy but also happens with Arnold, the female pair is always bring that up in a attempt to piss off the make character)
About Turner the teeth are supposed to be ugly, of course he doesn't really look ugly to us, because that'd be probably not appealing to watch, but again it was constantly brought up as a fact in the program as if there was something wrong with it (not perfect) and who knows (don't remember in what way specifically
If it's you getting pissed off about the giant head, that's standard for a lot of cartoons so they can emphasize facial expressions without needing to zoom in so much the body can't fit in the panel.
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the point is that in these specific series I mention, the only character with such a trait is the protagonist, and because of this it is sometimes sneered. Whereas this characters that you show me, have both big heads, which means that it is not a distinctive trait, the same for Timmy turner, he has big head, but it is not his distinctive trait and what he gets mocked for. (because everybody in that series have a big head)