"You be looking good is improper ghetto slang English and sounds rather silly. The correct term giving a compliment would be "You look good."

"You be looking good" isn't considered proper grammar. The correct phrase would be "You look good", depending on the situation. "You look good" would be said in a present tense, such as, "You look good in that dress". "You are looking good" is very similar and pretty much means the same thing, though it's a lot more inclusive. You would say "You are looking good" when someone looks nice generally, but if you wanted to be specific, you would say "You look good in those shoes", or something similar.

I think it's just a slang way to say you're looking good. Or like with more emphasis. I'm not sure grammatically it's correct or wrong.

This is actually "bad" English. Either they are trying to be cute or just flirting with poor grammar. It should be "you are" not "you be". But, like the others have said this is slang.