He is boss of a whole bunch of people who were in that stadium. He can wear anything he wants. Also, “Elections have consequences.”
-Barak Obama
There is no question that President Trump is the boss of everyone on that field, be they Democrats or Republicans or something else. And indeed, he can wear whatever he wants. And he knows that many will not like the choice to wear a campaign hat. The question is, I think, WHY? Why choose a campaign hat? Why not instead choose a super cool hat with the presidential seal, or maybe, a specially made hat for the game?
On a side note, President Obama also, when he was president, was the boss of all those players and hopefully, when he WAS president, those players knew how to spell his name.