showing the world from a kid's perspective.
##This, I think is the main reason, I remember the world feeling kinda weird and scary when I was a kid and I think for the most part, this is Asano's way of capturing that feeling.
Also, I believe in some cases a character's face is designed to represent their personality, allow me to demonstrate:

Look at how stupid these two women look! this is in chapter 47, this woman is saying "To think my son got in such a prestigious school!! It's a tragedy that I can't brag about this to every Japanese on Earth." and right before she says that we read "Punpun was not particularly happy.".
Asano shoves this woman's annoying behavior in our faces putting us in Punpun's shoes and showing us what seeing this through his eyes must feel like.
You could also make the argument that in this manga we're seeing everything through Punpun's eyes and this is not what this woman actually looks like... but <b>she might as well do</b>, you know what I mean? she might as well look like a complete fucking clown saying stuff like that.
This would also explain why every imagery in this world is so weird, maybe all of this is Punpun's understanding of reality instead of reality itself.
P.S: This isn't something that only happens in the beginning remember how displeasing Pegasus looked like? he had boogers coming out of his nose! this is something that happens throughout the manga.