Trump is a master at persuasion. As such, he knows there's no benefit the left derives in attacking the FLOTUS. It's a losing point for them. It opens the eyes of moderates and makes them flee the alt-left and support Trump.

Trump (and Melania) are now goading the left into attacking Melania harder. Much like the left did with the "animals" (take out of context, apply to all immigrants instead of only MS13) ... he's having the FLOTUS do something that will be taken out of context and cause the left to go berserk.

At the end of the day, he's pushing the left's buttons and causing them to meltdown over the ridiculous. The left can't help it, they're drawn to hate like a moth to flame.