Would Tim Allen be ok in the film Mrs Doubtfire?

He probably could have pulled it off, as easily as Robin Williams......

after all, he underwent heavy make up transformations for  The Santa Clause.......so it wouldn't have been much more of a stretch to make him into Mrs. Doubtfire , either. 

Lots of movies are like that......where the actors that are so famous for the role NOW........were actually 2nd , 3rd or 4th choice for the lead. 

Tom Selleck  turned DOWN   Indiana Jones. 

Buddy Ebsen, was the ORIGINAL  Tin Man, in the Wizard of Oz.........but turned out,  he was allergic to the silver face paint.....and had to be replaced. 

Other actresses turned DOWN  Pretty Woman,  because they didn't want to play a prostitute.   The list is endless,  of stars who turned down  career making roles........only to see their replacement, go on to win Oscars  and  achieve  A-List stardom , while their own careers  tanked after they chose to star in some  total dog of a movie, instead. 

Possibly yes...................

On the comedic side, maybe.  Tim Allen excels more when the comedy requires a bit of slapstick though, and Mrs. Doubtfire didn't really have that.  I don't think he would have done as well with some of the more serious, sweet scenes.  Robin Williams was always known for his comedy, but actually had pretty good range as an actor.

I don't see any reason for the other members of the cast to have been different in your scenario.

NOBODY would be ok in that horrible movie.