If you go further back, like anime and manga from the 60's and 70's, you'll find it used to be quite rounded and cartoony! The current style has some things in common with the likes of Doraemon (and, yeah, Osomatsu-kun/Osomatsu-san).
I don't like the over-the-top "zany" stuff when it's too much, but I love how expressive everyone is, subtle body language when it's there (Mallow and her mom ;___;), and the disrupted lines to make things look softer (Rowlet's feathers when being stroked, Snowy's fur when it was first hatched, Lillie's hair at times...).
I also really love when the Rockets are drawn in more mature styles!!
Once the show's art style settled into more uniform and, honestly, kind of formulaic expressions (outside of very creative head animators like Iwane-san and Tamagawa-san, who always had their own distinct styles of dtawing, shading, etc.), we never got to see expressions that really pushed things past Kanto (it might look more "serious" for the more angular 90's style, but if you go back and watch, especially comedic episodes or ones where the characters freak out a LOT like when Ash met Gengar and Gastly in Lavender Town, creative sight gags and wacky expressions were in Kanto too! It's just not as heavily used).
The really nice thing about how free it is, isn't just the wacky faces, but getting to make more detailed expressions even in serious moments (Lillie's fear was often drawn in comical ways, but this look of genuine fear was one of the earliest hints it was something serious...).
So seeing expressions this rich really makes SuMo stand out. The artists all seem to have more freedom!