If you're doing something where you're developing transferable skills (if, for instance, you worked as a student ambassador, you would develop public speaking skills) and you do not work so many hours that your academic work suffers, then I think it is an excellent thing. I would not have got my grad job if I hadn't worked during my degree.
If, however, you are doing the same [sort of] job that you've been doing since you were 16, then there is rather less value in it, and I would advise you to look for something a little more stretching / relevant.