Because they are choosing to mock, not share a joke. And mocking is what we do when we try to humiliate and put a person down. A shared joke and a more run remark is different - eg with the starry top, a nice person who liked you might say 'You look like a star today!' Whereas (what I am guessing is) a disparaging tone and a remark that meant a criticism of America, is different. Eg, if you had American relatives, you might say 'Ah, representing the overseas branch of our family today' or something.
It's the tone, and the intent of mocking, that makes for a shitty remark. Add in facial expression and body language. So little is about the actual words chosen. It's not what you say, it's how you say it.
I could say to someone 'How DO you find the time to have your nails done? Please tell me, I am shit at time management. Your nails look lovely.'
I don't know what the answer is. I might be tempted to respond with, Did you know how mocking and negative you sound when you say things like that?
OP - none of those you mentioned were subtle remarks. How people see us being treated, and what they see us seeming to accept (which is NOT the same as accepting) tells them how to treat us, and in the case of our children, our children too.
Is your dd's boyfriend going to think, Well, I can say what I like about her mum because she seems to be OK with the criticism, even her own mum clearly doesn't think she is great.
It's really important to surround ourselves with cheerleaders, and not doom fairies. You know - the critical misery mongers, jealous energy vampires, furious attention grabbers.
Easy for me to say, but yes, you are right to avoid your mother this Christmas. And every day, tbh, because she won't change. Grey rock her.
Also, the critical comments are a jealous twit noticing how nice you might look, and grabbing the attention to themselves. Eg, noticing your hair is looking nice and then saying a) I thought you hadn't gone to the hairdresser yet and b) look at my lovely nails! A put down followed by a spiteful contrast, demanding attention for her nails.
The point is, these remarks are made because your mother HAS noticed. And she is pissed off about it. She is upset when she perceives you doing better than her. What does that tell you.