That bothers me as well. In part because I don't think they would make that sort of comment to someone who is not trans. Though I'd be bothered about them talking about my appearance in general (whether saying something positive or negative), unless maybe if it was something that I brought up.

I think of once, early on in seeing my T, when I talked about the amount of beer I was drinking at the time. He replied, "How do you stay so skinny?" (Beer is pretty high in calories.) I didn't say anything at the time, but it bothered me. (Also, I'm not skinny!) He hasn't said anything about my appearance in years (I'd call him out on it at this point if he did). But a couple years ago, he did describe a former (female) client once in rather unflattering terms. I told him that did bother me, and he apologized. Hasn't done it since.