In Act II and Act III we see that Mercutio does not like Tybalt. In Act II scene 4 Mercutio and Benvolio are in the streets of Verona discussing Tybalt and mocking his arrogance. They know that Tybalt is mad at Romeo for disrespecting the Capulet family and wishes to challenge him to a duel (they know this because Tybalt sent a letter to the Capulet house issuing the decree for the duel). They wonder if their lovesick friend (they don't know about Juliet yet) will be able to focus and handle himself enough to accept and win the duel.
Mercutio describes Tybal to Benvolio
More than Prince of Cats, I can tell you. O, he's the
courageous captain of compliments. He fights as you sing pricksong, keeps time, distance, and proportion;
rests me his minim rest, one, two, and the third in your
bosom! the very butcher of a silk button, a duellist, a
duellist! a gentle man of the very first house, of the first
and second cause. Ah, the immortal passado! the punto reverso! the hai!
It at first appears that he is praising Tybalt. He explains that He Tybalt is very skilled and fights by the book. He knows all the Italian names for the fencing moves and does all the expected rests. However, as he continues, he explains that he doesn't like those people who spend too much time studying how to fight instead of those who actually know how to fight. Why should they have to listen to these men who complain so much about the right terms and manners and then can't put up a good fight.
Why, is not this a lamentable thing, grandsir, that we
should be thus afflicted with these strange flies, these
fashion-mongers, these pardona-me's, who stand so
much on the new form that they cannot sit at ease on
the old bench? O, their bones, their bones!