I disagree with some of the folks who are gibbering on about how "testosterone is irreversible" and such. I think you're giving troons too much credit. Testosterone doesn't make you auto-ugly. Many drag queens look fabulous and feminine despite not taking hormones etc. Being male is no excuse.
Troons are ugly because they put literally zero effort into their appearance, and a big reason for that is because they are men. A lot of women spend over an hour in front of the mirror every morning, doing makeup and styling hair -- in addition to all the other appearance routines, saving and waxing and plucking and exfoliating and conditioning and whatever. Since they've lived their entire lives as men, troons haven't developed a lifetime of these grooming habits, and are used to just rolling out of bed and slapping clothes on. The lipstick is for "fun" because I'm "expressing my gender today", and isn't seen as a social requirement. I'd say a lot of them are equally as ugly as some ugly women who also have poor grooming/hygiene habits. Actually, men are gross too, but socially are less arguably less punished for it.
But, that doesn't' explain why troons in particular have this problem, since there's lots of shabby schmucks of other flavors. I argue it's honestly another aspect of their narcissism, professional victim-hood, and lack of self-awareness. They don't think they should *have* to put any effort in, because they are just as good or better than you regular plebs, any problems with their appearance aren't their fault, and since they *identify* as a Beautiful Woman you need to treat them as such, you bigot. After all, "beautiful woman" is a social construct and they can redefine it to mean lazy troon. Everyone else needs to change to suit me, so get with the program. (Also, I'd say there's a good dose of ego-protection in there.)