Continuity mistake: When Kenan and Kel are in the kitchen trying to make the birthday cake, there is a blackboard behind them. The writing on the board disappears and then appears again during the scene.
Continuity mistake: When Kel initially hits the wall with the mixer whilst they're making a cake the picture falls down, however when he chases Kenan over to the same wall with it seconds later, the picture is back on the wall again. (00:14:35)
Answer: He's simply enjoying doing it himself. Especially since he likely believes he can do it better than she can.