Image description: 4 panel comic titled If Trans People Said the Things Cis People Tell Them.
Panel one has an androgynous individual talking to a woman. The androgynous individual, "To me, you are 100% a girl, you know?" The woman has a look of confusion and disgust and responds with, "Wow, thanks."
Panel two has the same androgynous individual speaking with a man. The androgynous individual is saying, "It's so incredibly you look just like an actual boy!" The man is rubbing the back of his head in nervous confusion and says, "Yeah, I... am one?"
Panel three has the androgynous individual speaking with someone of ambiguous gender. The androgynous individual says, "I would never have guessed you were cis! Congratulations!" The other person is looking at them with an expression of confusion and concern.
Panel four has the androgynous individual speaking to a man. The androgynous individual says, "I think you are so brave for being who you are." The man is frowning at them.
The comic is credited to @assignedmale on Facebook, Twitter and Tumblr and SophieLabelledraws on YouTube. Their merch is at assignedmale.etsy.com and Pateron is at patreon.com/sophielabelle
I'm sorry, is saying to a trans person that they did a great job passing, rude? I thought it would be validating.
Yes, it's rude, because of all of the things it implies. It assumes our goal is to hide our trans identity, as if that's something we all want to do. It assumes that looking cis is better than looking trans, because otherwise it wouldn't be a compliment.
In short, whilst it's unlikely to hurt the person you're saying it to, it sustains a lot of ideas assumptions that hurt all trans people
Thank you for asking, and the key is "you look good" (stop here and do not say "...for a trans person").
If the shorter version of the compliment feels insincere to you, just don't offer it.