I think context and intent is what's important in addressing whether dragon queens are good or not. Would someone dressing up as a drag queen in support for transgender be harmful? Context and intent matters where as dressing up in a sense to mock transgender is a whole different issue. I don't think you can make a blanket statement regarding it.
I don't think anybody doing drag is trying to mock trangendered people.
One of the more important queer theory ideas is that gender is mostly performative, both in the sense that who you have sex with doesn't depend on your genitals, and in the sense that the patriarchy projects a lot of behavioral expectations on you based on your genitals that aren't actually fundamental in any biological sense.
A lot of queer groups, behaviors, etc. are interrogations of this idea -- taking some section of patriarchal male/female behavior patterns, whether you're slicing in a traditional or nontraditional sense, and expressing your personal gender using those behaviors. Arguably it's even central to being transgendered (though obviously transgendered people are born with a gender that doesn't match their body).
Drag queens are just a pretty aggressive exploration of this concept. If gender is mostly performative, then you should be able to put it on and take it off like a wig. That's why RuPaul is a she in the dress and a he in the suit.
I don't think it really relates directly to being transgendered at all -- one is something you're born with, one is a profession/lifestyle you adopt.
I agree that if this is a Trans Pride event then cis drag queens maybe aren't super appropriate?