Some people might say that the characters in Phineas and Ferb only look strange to viewers, but that in-universe they look "normal" to each other. However, the series has shown that the characters are actually aware of their strange body shapes and a lot of other things too.
Sometimes they will comment on Phineas' lack of a chin or his triangular head. The same thing also can be applied to any other character in the show. Sometimes characters look so different from each other that it looks like they belong to different universes.
So, my question is, how does Phineas put his shirt on? Is his shirt that stretchy? If that were the case, wouldn't it wear off eventually, anyway? I know that both brothers have a lot of the same clothes to change, but I think that is too simple an explanation.