“The crowd is a monster.” Does monster work in the scentence? Monster is singular so I don’t know I’d it work. I’m doing a personal story for myself.
Yes, it absolutely works.
"The crowd" is singular anyway. (In British English it could also take a plural verb, if you were talking about the individuals in the crowd, eg "The crowd were angry". But here you are talking about the crowd as a whole, so it is singular).
And even if it weren't singular, there is no requirement that the two sides of an verb like "is" or "becomes" have the same number, eg:
All those mistakes are an embarrassment for the board.
Freedom is many things to many people.
(Note that the verb agrees with the subject, not necessarily with the complement).