Is Mercutio a good friend to Romeo?

Yes, Mercutio is a good friend to Romeo. One of the most touching details of the play is the strong friendship between Mercutio, Romeo, and Benvolio.
If Benvolio is the steady, level-headed friend, Mercutio is the party guy, always fun to be with because of his energy and wit. On the way to the Capulet party, Mercutio shows his friendship by trying to talk Romeo out of mooning over Rosaline—and he tells a very lively story about Queen Mab. Mercutio tells a cynical story because he knows Rosaline simply isn't that into Romeo, and he wants his friend to get over her.After the party, when, unbeknownst to Mercutio and Benvolio, Romeo has fallen madly in love with Juliet, the two friends catch up with the missing Romeo. Romeo is now perked up and alight with joy, and he matches Mercutio witticism for witticism as they battle it out with words. Here we get an insight into how the friendship must have sparkled in days gone by.
We know Mercutio has been a good friend to Romeo, too, when Romeo risks everything he has with his beloved Juliet to avenge Mercutio's death by killing Tybalt.

It could be argued that Mercutio is not a good friend to Romeo, and there is a lot of evidence from the text to support this idea.
When Romeo is depressed about Rosaline, Mercutio does not offer much in the way of sympathy or understanding. In fact, he says this to Romeo:

If love be rough with you, be rough with love;Prick love for pricking and you beat love down.

In other words, he is telling Romeo to forget about Rosaline by having sex with other women. He is, therefore, not a good friend in this instance because he is far more interested in having fun than in listening to Romeo's problems.
In addition, in Act III, Scene I, Mercutio proves himself a bad friend to Romeo by fighting with Tybalt. When Romeo tells Tybalt that he loves him and does not want to fight, Mercutio does not show any support. Instead, he calls this a "vile submission" and goads Tybalt into fighting. Moreover, when Mercutio is killed by Tybalt, he curses both Romeo and his killer. This is not the sort of behavior that we would expect from a friend.
However, looking deeper, we see that there is some genuine friendship between Romeo and Mercutio. If there was not, Romeo would not avenge his death by killing Tybalt and, therefore, risking his relationship with Juliet. Moreover, by fighting Tybalt himself, Mercutio saved Romeo from injury or even death.
Mercutio's role in the story is to act as Romeo's foil or contrast. Mercutio provides some comic relief when Romeo is brooding on Rosaline and some action when Romeo does not want to fight Tybalt. Mercutio and Romeo are, therefore, supposed to be different, and this should not detract from their clear sense of friendship.

Mercutio is a good friend to Romeo, but there's a distinct lack of depth to their friendship. Mercutio is an incorrigible wit, and his cynical sense of humor and partiality to sparkling banter make him a fun guy to hang out with. But because Mercutio has such an anti-romantic view of love, he can never be a confidant to his friend. Thanks to his undying love for Juliet, Romeo is experiencing so many strange and wonderful feelings. Yet he can't really share them with Mercutio, as he knows that he'll just treat the whole thing as a big joke. It's telling that Mercutio doesn't know about Romeo's marriage to Juliet. Just imagine how he'd react!
Nevertheless, Mercutio shows his friendship and fierce loyalty toward Romeo in his fatal fight with Tybalt. Romeo is not prepared to do battle with Juliet's cousin, so Mercutio bravely steps into the breach to fight for his friend's honor. His subsequent death is a mark of how much Romeo meant to him, and how he was prepared to sacrifice his life for Romeo's good name. What friend could do more?