It's so funny isn't it? or it's very funny isn't it? マジウケるっしょ！ マジウケるっしょ？ マジ is actually “serious” or “really” but in this case, I think it would be “so” or “very” to emphasize being funny. お前マジかよ？ マジかよ？ Are you serious? or Seriously? マジで？(本当に？)　マジですか？(それ本当？) Really? Is that's true? ウケる is funny. Sometimes it could mean someone is popular among particular people. お前マジウケるよ あんたマジウケる You very funny! Note : these phrases are pretty informal Some Japanese people might say マジウケろっしょ! in that case, Sheena-san's answer would be right.

http://zokugo-dict.com/03u/ukeru.htm マジ is used here as an emfasis. うけろ is an imperative form, so it directly means "laugh!!" or "find it interesting!" ーっしょ。Its hard to trasrate to English but it sounds like that this person is requiring consent of an other person. personally I don't recommend you focusing on this one. To sum up, it means "laugh" btw is it really うけろっしょ？ honestly it sounds wierd to me. I would say まじうけるっしょ。which means it's very interesting, isn't it? and if I want to say "laugh!', I'd say うけろよ。orうけろや。 ex) If I said some jokes and you didn't laugh, I would say おい。そこうけろよ。 this sentence is very informal though.

マジウケる means "to be amused" or "to find something interesting", I think. ^_^