The same as any difference between present simple and present continuous. You are nice = You're a nice person (most of the time). You're being nice = You're behoviour right now is very nice. E.g. You get a bunch of flowers so you say: "You're being too nice."

"you are nice" is the way you talk and feel about other that is comfirmed "you are being nice" is tend to "you are trying to be nice" " you are doing something in nice way" etc. if you say this sentance, it`s not really good to hear though. because it sounds more i wasn`t show nice enough to be commited.