Mercutio unwittingly causes two major things to happen in the play. In Act I, Romeo is feeling a sense of foreboding about going to the Capulet party because he had a bad dream. In the dream, he went to the party which set off a chain of events that led to his death. Because of this feeling of unease, Romeo might have backed out of going to the party if Mercutio hadn't given his Queen Mab speech about how dreams mean nothing more than a failry queen's whispering in a sleeper's ear. By convincing Romeo that dreams are meaningless, Romeo does go to the party, and a chain of events IS set off with he meets Juliet. In Act III, Mercutio is pretty pugnacious and looking for a good fight. When Tybalt comes to challenge Romeo, it is Mercutio who continues to taunt Tybalt even after Romeo refuses to fight. If Mercutio had keep his mouth shut and his sword in its sheath, then Tybalt probably would have walked away, just calling Romeo nasty names over his shoulder. Romeo would not have killed Tybalt if Tybalt had not killed Mercutio. By Mercutio instigating the fight, that also sets off a chain of events that lead to the catastrophic conclusion. Romeo avenges Mercutio's death by killing Tybalt, he gets banished when Benvolio lies for him and claims that Tybalt started the brawl, and he kills himself when the Friar's plan, like "the schemes of mice and men", goes terribly awry.
Mercutio is a significant character in Romeo & Juliet as he is possably the most GAY person in any play of that time!
It is also so obvious that Mercutio is gay as he loves Tybalt, but Tybalt and Romeo are having an affer, this causes his dying words to be 'a plague on both your houses' as he damns them forever!