Act V of this play is funny for a number of reasons. If you're writing several paragraphs, you could spend one paragraph on each area.
The first area is simple relief. The four young lovers have been through so much—up, down, forbidden, allowed, etc.—that they are laughing because they aren't in pain, and we laugh with them.
The second area of comedy is the way that other people make little side comments while the rude mechanicals are acting. It's like watching a movie with friends and poking fun at it.
The third area of comedy you might talk about is simply the bad writing and acting of the play that the rude mechanicals put on. The play is overwritten, and the mechanicals are so unimaginative that that have to explain everything. It is like watching little kids tell jokes.
(Oh, for a general explanation of the act, you might check out that section of the enotes study guide.)