Robin Williams was known for his ability to improv. If you ever saw him on talk shows or other unscripted venues you could always see him firing off jokes like a machine gun. The man had skills. He could also "go blue" fairly easily, often requiring those unscripted performances to bleep him if they weren't in a position to allow adult language. Williams has talked in the past about filming movies and occasionally having to dial back his performance. But rumors have persisted for years that the actor went so far while filming Mrs. Doubtfire in the early '90s there was actually an NC-17 version of the family film.
A recent viral tweet makes the statement that Mrs. Doubtfire had up to four different cuts of the film, each with different MPAA ratings from PG to NC-17, and all because of Robin Williams tendency to improvise scenes. No NC-17 cut of the film has ever surfaced, so it's unclear at best if such a thing exists. Co-star Mara Wilson was once asked by People if an NC-17 cut of the film could be out there. While she wasn't able to confirm its existence, she was at least able to confirm that Williams did occasionally say stuff during production that would not fly in a family movie. According to Wilson
I don’t know about NC-17, but with some of the things he said, I’m sure there was probably an R [rated] cut somewhere.
While Robin Williams certainly may have gotten a little "mature" with improv choices, that's not the same thing as saying an NC-17 cut of the movie exists. A "cut" implies that a film has been edited together in a more or less complete way, and that seems unlikely under these circumstances. Nobody was ever going to release an NC-17 Mrs. Doubtfire, even making such a thing as a home video release would have an incredibly niche audience. While the content for such a film might technically be able to exist, that's not the same thing as saying it does.
And when you think about it, there probably aren't many complete scenes that include Robin Williams getting vulgar. The moment he went too far the scene probably broke down, so what's far more likely is that there may be a collection of NC-17 worthy outtakes.
But in the end it seems unlikely that there would even be quite enough of that adult content to even make a whole film worthy of such a restricted rating. In Mara Wilson's comments on the potential for such a movie, she says Robin Williams was careful not to go too overboard when the child actors were around. Wilson continued,
He was very respectful around me. He had kids himself and obviously he’s known for some of his blue material; he didn’t do that around us.
If he kept himself in check in all the scenes where the kids were around, then the number of scenes where he could have gone full NC-17 become more limited. Even if you took the absolute most vulgar version available of every other scene, I wonder if you'd really have enough for an NC-17 film since so many scenes wouldn't be that bad.
While some mature moments wee almost certainly shot, it's unlikely we'll ever see them. And even if there really as NC-17 cut of Mrs. Doubtfire out there, there's even less chance we'll see that.