Considering the existence of TVDinner items and the microwave in the room next to the TV and the fact Tenna's display colors are kind of messed up makes me think that the whole Mike shit is indeed an imaginary friend, but it's also Chef Mike the microwave fucking him up. That little change in electricity when a microwave goes on and off made Tenna schizophrenic for Mike. And chances are that after the family fell aapart, so did the need for the microwave since seemingly everything Toriel cooks are pies.
So in truth, there never was any Mike. Tenna is just unhinged.