Well, I'm not interested in labeling you a cunt, but I think I see where the remark is coming from, and I know what it's like to say something on the internet and now there's just a firehouse of criticism with a lot of it not being in great faith, and I can sympathize with that.
I don't know how reflective the examples you gave are of things you actually say to people, but if you think you know that someone is objectively beautiful or smart and that you telling them that is some kind of bandaid to rip off for them - that is something to reflect on, these are textbook examples of socially constructed ideas. Beauty is in the eye of the beholder, as they say. You're smart or beautiful when your community regards you that way, it's the consensus of tastemakers. It's nature as consensus can make it feel "real" or objective, but these are vague squishy concepts that change across time and across cultures. Even within a culture there's a huge diversity of preferences.
So some communities might prefer technical skills and call people smart when they're great programmers, some might value media criticism, it's not set in stone. Same goes for beauty, but I'd wager there's even less consensus there.
With that in mind, telling someone you don't find them beautiful is just placing a burden on them. It's telling them about how they fail to measure up to some measure in your mind, but the only outcome here is that either don't care or it bums them out. If you don't find them beautiful - that's a property of your eyes, not them, and you're entitled to feel that way but it's something to keep to yourself.
The thing you said in another comment about people being invariable reminded me of a comment I made recently:
https://news.ycombinator.com/item?id=35318596
