In Act I, Mercutio shows himself to be witty and eloquent, and he is also a good friend.
Mercutio is a good friend, and a jokester. He may tease Romeo, but he also has his interests at heart. He is interested in Romeo’s life, and tries to get him to move on. Mercutio is also somewhat of a dreamer. His Queen Mab speech is both joking and flighty, but there is some sentimentality to it as well. He is witty and charming, and while it is tempting to discount him as just comic relief, he actually does say relevant things.
Consider the speech from secene IV
True, I talk of dreams;
Which are the children of an idle brain,
Begot of nothing but vain fantasy;(105)
Which is as thin of substance as the air (enotes etext p. 28)
Although it is goofy and contains puns, this speech is also introspective and foreshadows important events.