Bair and Fair are not good options because of their hitbox locations and the fact that you can SDI out of them pretty easy (especially considering Pika is falling with it). Dair is ok, just kinda slow. You won't have Dair guaranteed unless you grab the edge while MK is lagging from his first Uair (like in the middle of it or right after the start, not near the end of Uair), and he also commits to the second one. Nair has a better shot at hitting him frame wise obviously, but position wise it's pretty hard. You can have a frame advantage on MK, but not be able to cover the distance to exploit it. Characters can get a frame advantage on MK from PSing his Uairs, but not be able to reach him in time for it to matter (which sucks). In Pika's case, honestly if he had a Dair like MK he would be the best choice in the game to stop planking, even better than another MK. He's got the speed (if you get in at the right time), but not the range, to stop it.