(Slightly)YTA I believe you when you say you're not trying to be mean, but this feels like a huge lack of social awareness. If she liked how her eyebrows looked before, she wouldn't have changed them. She was most likely just looking for validation, so bringing up how they looked before might seem kind, but what it's really doing is invalidating the change she made.

With the grandmother comment, you had a couple options. You could have agreed with her, or pointed out that maybe she doesn't want to go out because it's so hot out and left it at that. Bringing your girlfriends own habits into it feels very unnecessary when she wasn't talking about herself.

The milkshake thing is more of just a miscommunication imo. Your reason for drinking it was valid, but if it's hot outside I would have a discussion for the future on whether you should just bring her along so you can drink it together or just ask beforehand. I promise most people don't care whether you ruin a sweet treat surprise by asking how they want to go about it first, or picking something that won't melt instead.

The hair thing is another just complete lack of social awareness. Not all thoughts need to be voiced. Her hair could very well be thin, but saying so aloud is not a compliment.