Disclaimer: all of this is within the context of us having a connection intimate enough that you value knowing my perception of you and vice versa. This does not apply to comments from non-intimates.
Part of the reason I have a hard time with my friends and lovers having a negative self-image is that it contradicts my perception of reality. And so, when I'm around them, I feel like I have to defend my perception of reality from them trying to tear it down or reshape it. I can't enjoy what I am experiencing as reality because the person that I'm with rejects the very existence of my reality. The way that many people feel when someone dumps all over their favorite show or book or movie is how I feel when my friend or lover dumps about themselves to me. It makes it so much harder to feel that same fresh excitement, because now there's this pain on top of it.
I do better with people who have an exaggeratedly positive value of their self worth because that's closer to how I see them, and it's erring in the right direction. My reality can easily stretch to encompass the idea that someone is better than they seem to me, but it breaks my reality to try to imagine that someone is worse than I perceive them to be.
None of this means that you can't share your feelings of low self-esteem with me, it just means that I need it to be framed as your feelings, and not as some kind of objective reality that you're trying to get me to accept. It's not an objective reality. It's not my reality, and your reality is not the only one that you are part of. If you are my friend or my lover, part of the fabric of my reality is you being a worthy person.
And frankly, I think it's just mean to respond to someone expressing happy feelings about something with telling them that they're wrong and the thing they like is terrible. If you think that the thing they like is terrible and they need to know, tell them at another moment, when you're not crushing their expression of happiness. Not that I am innocent of this!
I know that I sometimes crush people's happiness, but I don't do it on purpose. I don't do it if I realized what's happening. I do it because sometimes my thoughts pop out of my head without me considering whether or not I should say them. And once it happens once, I try to set a memory tag on that topic so I don't react that way again. That's all I ask of my friends, too: just try not to respond to my expressions of admiration by crushing them.
If you need to laugh it off by saying "ah yes I am the most enchantingly beautiful being ever to live" in an exaggerated manner that will be totally fine with me! I would actually love that even though I know you're not being earnest, because it feels playful and I can respond to it happily. Other good choices might be if you want to say "I'm glad you feel that way" or "okayyyyyy" or "there's no accounting for taste!" with a smile or just be silent in response, none of those things will crush my expression of happiness.