Yes, "pull off" generally means "to succeed in doing something" - in a fashion context it means succeed in putting together an outfit or a "look" that works or looks nice or appropriate. "Pull off" more literally means to take off a piece of clothing, so you have to be careful here. When you say "pull off this denim coat" as opposed to "pull off a cowboy look" it sounds like you are literally talking about removing the denim coat (although denim jacket is better than denim coat here). You might say This denim jacket is not enough for me to pull off the cowboy look. I wouldn't be able to pull off looking like a cowboy even if I were to wear a denim jacket. I hope that is useful.

Hi Yes in this context "pull off" can be used. It doesn't really refer to if the clothes fit in a physical sense, but more that they suit your personality. But I guess there is a relationship between physicality and things like confidence. It can also be used for things that are not about clothes, but about appearance. For example "He tried to trick the school by pretending he was a teacher, but he couldn't pull it off." = "He tried to pretend he was a teacher but he was not convincing." Be careful with "pull off". Like many phrasal verbs it has multiple meanings in different contexts.