And he should be allowed to dress as anybody he wants to, as long as it's not in the intent of disparaging. If someone decides they will dress up as a character who happens to be of another skin color at halloween, and it's done in good spirit, then this is totally fine - yes, I agree, actually.
The issue are the droves of idiots, that spend their days trying to come with things to complain about. Just part of the outrage society we live it - but Trudeau is one of those droves of idiots who find offence in everything.
So, while I agree with your first point about freedom of expression, I'm a bit glad that Trudeau is catching some flack over this.