First of all, Vin Diesel is the damn man. Half-baked, nobody gives a shit about? You obviously haven't realized the greatness of Vin... but you will.
On topic though, I'd say Vin Diesel draws more eyes to Mania and although you would rather see somebody like BNB or Ryback, there are probably a ton more casuals that would tune in for Diesel, especially if The Rock were involved. I get you're argument that a mid-carder is having his shot taken away in turn for a celebrity but shit, when has that not happened? Mickey Rourke, Snooki, Mr. T, Lawrence Taylor, Akebono, Floyd Mayweather, and Butterbean have all played small-large roles in the ring at Mania... why not Diesel? The guy obviously fits the bill better than almost anybody else. Besides, it's not as if somebody like Bo Dallas or Fandango REALLY has a chance of getting over, do they?
Usually I'm opposed to celebrities in the ring, but if Vin could add something to HHH/Rock by getting a few good spots, why not? Apparently the FF franchise couldn't pass up the opportunity to pair up The Rock and Vin Diesel, WWE might want to think about it as well. To OP, this is nowhere near a terrible idea. In a business sense, it would bring in a shit ton of money but you're talking to the demographic of wrestling fans that actually value wrestling above all else... I could see how some people would be opposed.