What does it mean when he says “you’re always over dressed”?

Anonymous
So my best friend is a guy and we always have lunch once a week. Because it’s been hot I’ve only ever been wearing sundresses everyday. Well today he said “you’re always overdressed! We’re just getting a sandwich.” My response “it’s a T-shirt dress, this is casual”. He kept staring, eventually he thought my purse that looked like a coffee cup was cool.

I feel bad now. What does it mean when a guy says that? Should I try to dress down a little more?
What does it mean when he says “you’re always over dressed”?
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