Shouldnt we first know the reason he is alive, before we judge it as stupid?

I have not read the comics. But as I understand it Minsc & Boo were turned to stone at some point after BG2. Then healed back to normal a little bit before Descent Into Avernus I believe(not sure how/why/when exactly).
Personally I don't have a problem with this. It is 100% possible to happen in this universe...hell we fight Drow that have been turned to stone for longer than Minsc & Boo in the Spectator fight in the UD.
No, it's much worse than this. The actual Minsc from the original games is dead. In Baldur's Gate, a statue was erected to celebrate Minsc as one of the heroes of the city. A surge of wild magic in the area of the statue caused the statue to come to life. *That's* who the current Minsc is! Seriously stupid.
Actually, it was confirmed somewhere that it is an ordinary statue and not a Minsc turned to stone for some reason?
You all should read the Descent into Avernus adventure and the Baldur's Gate comics: Minsc and Boo were indeed petrified somewhere around 137X D.R. (can't remember the exact date). So the ''statue'' that had supposedly been comissioned to put in Baldur's Gate for everyone to see was initially Minsc and Boo, petrified. The ''statue'' stayed there until 1480 D.R. when it was unpetrified by a wild surge. The ''statue'' has since been replaced by a real statue.
So yeah, the Minsc we'll probably meet in BG3 is the genuine article.