The mood in Act 2, Scene 4 can only be referred to as humorous and playful. This scene portrays the friendship between Romeo, Benvolio, and Mercutio and is a classic "boys being boys" scene. The humorous and playful mood is portrayed through the characters' jokes and witticisms, which can even be seen as lewd and crass.
Mercutio is especially guilty of lewd witticisms seen in his sexually implicit puns. For example, when Benvolio and Mercutio first see Romeo after he had given them the slip at Lord Capulet's house, Mercutio makes lewd jokes containing puns in connection with his assumption that Romeo had been with Rosaline. One such joke containing a pun can be seen in the lines, "O flesh, flesh, how art thou fishified!" (36-37). To be "fishified" literally means to be turned into a fish; however, fish also give off an odor. Hence, this pun is also a sexual innuendo.
Mercutio continues to set the playful, humorous "boys being boys" mood of the scene when he begins to insult the nurse. One example can be seen when Nurse asks Peter for her fan, and Mercutio declares, "Good Peter, to hide her face; for her fan's the fairer face of the two" (98-99).
Hence, we see that all throughout this scene, the characters create a playful and humorous mood through their jokes and insults, just as boys often do.