I had to figure out how to tell my husband that his bottom teeth have gotten horribly stained. It looks like he spits tobacco or something (apparently the culprit is a bag of tea we bought a few months ago, which he's been having in the afternoons). I said, "You know I love you no matter what, but did you know that your teeth have gotten really stained? I just thought I should mention it in case no one had told you." He said his business partner had actually said, "You need to get your bleeping teeth cleaned." So he knew, apparently, lol. He did start brushing better and they're like what people's teeth looked like in the 80s.
Your partnership is based on mutual attraction. It's OK to mention (gently, obliquely) that you prefer a certain look over the one that's being adopted now. Most people want their partners to be attracted to them and will take it under consideration. Just be kind because at the end of the day, it's just hair, and if it makes her happy then that's nice, right? My husband shaved off his beard once and I was like, oh, hmmmm, yes, well, do YOU like it? He got the hint and grew it right back.