its called that because 64+63=127

Actually this game is heavily inspired by Super Mario 63, a flash game made in 2009. That game was basically 2D Mario with FLUDD as well lol, I'm guessing this game is trying to be a sequel to that game.

There's an official answer to this by the team:

63 used the formula [ N - 1 ], so SM127 uses the formula [ (N * 2) - 1 ]

They just followed the jig of how Super Mario 63 subtracted 1 from the original Super Mario 64. I guess they multiplied 64 by 2 (to symbolize this being a sequel) and did the same remove 1 thing, ending up with 127.