Disclaimer: nobody has directly called my ugly to my face recently. I'm just using myself as an example.
1 - Nobody is ugly.
It's true. I agree, some people reeeeeally need to pluck their eyebrows--and if they did, they'd be a lot prettier. Or if they weren't wearing orange lipstick and the wrong shade of foundation, that would help too. But as for the big picture, if you can ignore that stuff, everyone is pretty. I never look at someone and say, "wow, she's ugly." I do look at someone and say "wow she needs to pluck her eyebrows" but that's another topic for another day.
2 - Who are you to judge that?
As if you're perfect yourself? Who are you to tell me how pretty or not pretty I am? Did I ask you? No, didn't think so.
3 - It's not a fact.
Everyone sees beauty differently. Someone who you might think is "ugly," someone else might think they're gorgeous. People say "she's ugly" as if it's a fact. But really, beauty is not black and white. It's not like math, where 2+2 always equals 4.
4 - They're getting hung up on one feature.
I'm sorry if I have a little acne, so shoot me! I'm sorry if I have a big nose. I got news for you, it's too big for me too. But that doesn't make me ugly. Clearly you're looking at one feature, when there are other things about me that are better.