Bat flipping is bad sportsmanship. This was never a thing until the self-esteem movement began playing.
Sportsmanship is something one should be learning at the college level. The coach should take him aside either after the celebration or after the game, and dress him down for putting on a show after hitting the grand slam, ensuring he never does it again--and if he does, pull him out of the game himself. Umpires should be calling balls and strikes, outs and safes, not trying to teach gentlemanly skills.