Actual unironic answer: skirts/dresses
Allowing character customization means its impossible to know how all clothing options will react to the position of the character and bike/ pokemon so the game designers made it so that the player will always look the same while riding.
Lets go solved this problem by having all outfits just be recolors of the standard boy and girl outfits so that the models for all the clothes are the same.
It sucks but honestly idk a way to fix it outside of maybe allowing the player to have an outfit set aside specifically for riding that doesnt allow options that would cause clipping. Similar to how you cant wear a hat with some hair options.
