It's not just Jean. Who else wears a bob haircut? Nadia Pym. For some reason, it's long in the Avengers comic, and that's not a case of growing it out after ANAD Avengers, considering how she looks in her solo Unstoppable Wasp book.
Face it, there are too many artists at Marvel that can't draw bob haircuts, that's why most ladies either have it longer or cropped short. Kamala Khan was actually conceptualised with a bob haircut as well (see here), but they ended up going with something a bit longer - which is inconsistently drawn, the length and whether she has bangs or not varying depending on the artist.