It's a gag, Oda made the guild structure be, 1st in command Buggy, 2nd Crocodile and 3rd Mihawk, while the powerscale is the other way around, showing that Zoro will also defeat a yonko level character without being a yonko character.
WG doesn't have a fight history for Buggy to place his bounty high enough, like other crews.
Mihawk's motive could be something like, Shanks might asked him to protect Buggy, which is not inconceivable, and if you think about it, in this chapter if it wasn't for Mihawk, Buggy might be dead rn.