Yes, it is a common informal term used as you correctly defined it. It is however used in a negative or pejorative context - when these athletic individuals have obnoxious personalities.

Generally, the term "jock" is used to refer to high school or collegiate athletes. It does carry a negative connotation. But as with all words, that also depends on how you use it (the context). If you want to avoid offending someone, it's better to refer to them as "athletes" instead of "jocks."

Jock isn't used in British English in this way. Jock is a term for a Scottish person, used by people from outside Scotland.

It's a very slangy term. It is usually not used for professional athletes, but people in high school and college who are on school sports teams. It's neutral or negative. Tone and context will tell you if it is being used as an insult or if it is just a description.