It is transphobic to say about a trans woman: “wow, she has better legs/makeup/hair/etc than me”.
Why is this a compliment? You would not say that about cis women. Angelina Jolie has better legs than you? Of course she does. Laverne Cox has better legs than you? Suddenly surprising. The implied second half of “she’s prettier than me!” is “…and I’m a real woman”. The implied premise is “all cis people are prettier than all trans people”, and so you assume that trans people will be happy to hear that you find them an exception.
Femininity and womanhood do NOT belong to cis women. Trans women and gender non-conforming femmes are not “trying to become women,” we are women and femmes.
Cis people are not the arbiters of gender. Trans people do not aspire to meet your cissexist standards, and do not need your backhanded validation. Stop being surprised that a trans person might have X attributes that are more attractive to you than you, a random cis person, who is not (gasp) a standard that everyone aspires towards.
(This post is an expansion on the small block-quoted idea of the many in this post by Alok Vaid-Menon on transmisogyny.)