Answer:
Mercutio thinks that dreams should not be taken seriously.
Step-by-step explanation:
In Act 1, Scene 4, when Romeo shares his dream with Mercutio and Benvolio after coming out of the ball. In this scene, the tips exchanged by Mercutio with Romeo reveals that he doesn't believe in dreams. He then gives his Queen Mab speech in a highly-eloquent manner.
He asserts that dreams are "the children of an idle brain."
With this, the readers know that Mercutio thinks that dreams shouldn't be taken seriously.