Written with capital letters as you have done, it looks like it's somebody's name. Without the capital letters it refers, in a snobbish way, to uneducated or unsophisticated people.

I'm guessing you saw this question on YouTube or some gossip website lol. Well, as stated above, "Riff Raff" must be some guy; so the question is asking if Katy Pery is having sex with this person. You should stay away from those kinds of sites, they kill brain cells.

Yep, I read this. when I googled it.

The Riff Raff I know of was the butler in The Rocky Horror Show.