Literally your personal opinion. There are prodigies around the world who train under others who are the best at their skill (games, sports, studies, etc) to surpass said trainer.
I gave you the reason for why Oda did it. Mihawk is not an "evil" antagonist with clashing ideologies. He didn't perform an evil act that which we know of unlike other villains, and he merely exists for someone to challenge him. Its not "weird" or wrong that Zoro asked him
Having Zoro train under him is not bad writing, in fact, it makes it 100% more realistic how a prodigy (Zoro) can surpass the teacher at such a young age.
Luffy literally studied under the 2nd hand to the Pirate King. Zoro training under anyone else would have not been nearly as convincing or effective. Nobody else has a goal that rivals theirs as a physical feat. It made sense that these were who they trained under during the timeskip.