I mean schools need funding for improved mental health counseling from somewhere, I don't see why a tax on M-rated video games is much different from a tax on alcohol or cigarettes. It's not like any of them are good for you, just they're bad to differing degrees. My only problem with it would be that while some games more than earn an M rating due to being needlessly violent (such as the Outlast series or Dead Space series), others are, well, more restrained (such as the Halo series) and get the same rating. The reality is that there is a huge difference between M rated games that isn't a big deal right now because they aren't really treated differently based on rating anyway, but if it started becoming a money thing I'd support separating these categories further (I think that more games should get the A rating just for violence, for example).
Of course, if this were passed on anywhere near a federal level, there would be a big push to get games under that rating threshold. Which I also would be fine with, honestly.
I also wouldn't mind seeing more violent movies get rated up and movies with language / sexual content rated down, because it's ridiculous that people are so scared of bad words or sexuality that they get placed on the same level as sadistic torture or mutiliation. But that wouldn't really happen with Trump's 'proposal'. Just the federal government would waste money on doing something the industry itself already does.