I can only speak for myself, I can't say "Well, you shold not use this or you should use that."
I don't like the name Angel as it strikes me as very cutesy and sickly-sweet. Also a bit tacky,it's a favorite of strippers and centerfold types.
But if you want to use it, I'd rather it be used as the given name or a nn for Angela/Angelina and like that, rather than just a nn for some non-Angelic name. When a girl named say, Brenda or Joanne is called Angel, it sounds a lot like when people call their daughters Kitten or Princess.