JJ - pré AI vs 50 BB stack ?

Well it is far from ok. You have a decent sample, in which villain 3 bets 4.68. As you are from UTG you cannot expext that he 3 bets all of that vs you. Here he does this either with KK+ or if you think he is wider than that - with QQ+, AK
It is a standard call of the 3b.
MP2 36.19% 35.98% 0.21% JdJh
MP3 63.81% 63.60% 0.21% QQ+, AKs, AKo

Raise or fold, there is no call here out of position vs mid stack. You have insufficient odds to set mine and trouble continuing postflop with an overpair against the villain's range here.

Fold and find a better spot against nitty 3bet mid stack. Half that 4.5 is probably resteals, so his true 3bet range against you is probably a depolarised classic 2.52 - AKo, AKs, QQ+.

Calling here is actually quite ridiculous.

Well, I didn'tsee the stack size, sorry. Indeed the call is worst. I prefer folding.

Vs random fish this would be fine. Here you have decent sample from villain and he 3bets tight. Probably he 3bets UTG opener even tighter. I would fold this one.

But if this guy is a professional shortstacker, then he must play this way (open-raise = pot-committed, 3-bet = super-pot-committed) with hands (e.g. 9♣:9♥:, J♠:T♠: ) that a regular player would simply call with - otherwise he'll go faster than Broomcorn's uncle (he blinded himself to death)?

This is more of a question than an opinion.