Yes. You can say "pull your socks up" to someone if you think they should improve the way they are behaving or the way they are doing something. Examples: 1.He'll lose his job as manager unless he pulls his socks up and starts doing a lot better. 2.The basketball coach said that I have to pull my socks up or I'll lose my spot on the team. You should only say this to someone such as your child, your student, your employee, or possibly to a friend. You shouldn't say it to someone like your boss, your teacher, your parents, or anyone you should speak to respectfully.

I'm guessing that's a British/Irish usage (I heard it from the nuns at school, but not since I've been back in America). I'm more used to hearing "straighten up and fly right", or "get his act together". "I'd better get my act together, before I get myself fired".