it really depends. From personal experience i don't get offended, however that won't be the case for everyone. Being transgender isn't anything more or less than just being a woman, so the notion that someone looks trans implies that all trans women are supposed to look one way, and can create shock value for those who don't fit the idea of what a trans person looks like. Id say the most respectful way of saying this would be to just tell them they're beautiful or that they "pass" very graceful♡