Here is the full list of all 21 Bears Can't Drift!? achievements.
Come first in a race or picnic battle
Activated Medium Difficulty
win yourself a hat
Activated Easy Difficulty
Activated Hardest Difficulty
Finished a race bellow 9th place
Launch a mortar strike on yourself
Steal the flag 10 times
Collect all the hats
In four player, finish a race 1st, 2nd, 3rd and 4th
Gold star every track in the Forest world
Gold star every track in the Arctic world
Gold star every track in the Ruin world
Completed the credits mini game
Get a top time on any track
Unlock every Achievement
Get a top time on half of all the tracks
Get a top time on all the tracks