Mercedes Javid must be a bad ass driver ... in reverse -- 'cause she wasted no time backing up the bus over her fired "Shahs of Sunset" cast mate Lilly Ghalichi.
MJ's not the least bit upset about Lilly getting 86'd ... telling us last night that Lilly had to because she broke the first rule of reality TV -- she sucked at being real.
As TMZ first reported ... Lilly got canned for being "boring" -- so maybe the problem was ... she was TOO real?
Whatever the reason -- you can almost hear MJ humming, "Ding Dong! The Witch is Dead" in this clip.