Short jokes?

This is what Trump thinks will be effective against the billionaire former mayor of New York?

And since when has Trump ever worried about fairness when not complaining about how everything was rigged against him?

I think it is pretty clear that Trump sees Bloomberg as an existential threat, perhaps an even bigger one than Joe Biden, due to his vast wealth and pledge to help finance the campaign of the eventual Democratic winner even if it is not him. 

I also take note of the fact that clearly Trump does not see Bernie Sanders as a threat, and appears to be working to agitate his followers in the hope that they will abandon the Democratic party if their guy does not win the nomination.

Which helped him quite a bit back in 2016.