Final answer:
The suggestion that Queen Mab "visited" Romeo reveals Mercutio's sarcastic and unsympathetic attitude towards Romeo's infatuation.
Step-by-step explanation:
The suggestion that Queen Mab "visited" Romeo indicates that Mercutio views Romeo's love sickness as fanciful and imaginary. Queen Mab is a character from folklore who is associated with dreams and fantasies. By bringing up Queen Mab, Mercutio is implying that Romeo's love for Rosaline is not based in reality but rather in his own imagination.
Shakespeare uses this reference to show Mercutio's skepticism towards love and his belief that it is a foolish and irrational sentiment. Mercutio's attitude towards Romeo's love sickness is one of mockery and disbelief.
Overall, the suggestion that Queen Mab "visited" Romeo reveals Mercutio's sarcastic and unsympathetic attitude towards Romeo's infatuation.