Girls call you an idiot and tell you to put on more clothes in cold weather. What does that mean?

First of all: the second fool may be your nickname. Generally, girls may give boys nicknames because they have a good impression on or like this boy.

Secondly, wearing more clothes in cold weather is to care about you, lest you get sick and catch a cold and be frozen.

Generally speaking, this girl either regards you as a good friend or likes you. You can use some pick-up routines to Doby her and see how she reacts. Maybe she's been together lately?