Where does Mercutio mock love in Romeo and Juliet?

Mercutio mocks Romeo for being in love twice: in Act I, Scene 4, and Act II, Scene 1.

In Act I, Scene 4, Romeo, Mercutio, and their friends are about to crash the Capulets' party. At this point, Romeo has not yet seen Juliet; he is still lovesick over Rosaline. He says he does not intend to dance because love makes him too "heavy" to dance. Instead, he will just hold a torch. Mercutio wants to get Romeo to dance, so he starts to argue with him.

Mercutio does not mock Romeo savagely in this scene, but he is not taking Romeo's lovesick state very seriously, either. At first, he suggests Romeo should "borrow Cupid's wings" to make himself light. When Romeo keeps insisting love is making him miserable, Mercutio says, "If love be rough with you, be rough with love" (line 27). By the end of the conversation, Mercutio is ready to "draw [Romeo] from the mire/Of—save your reverence—love, wherein thou stickest/Up to the ears" (lines 41-43). Here, Mercutio compares Romeo's "love" to mud, or perhaps something even more unpleasant. 

In Act II, Scene 1, Mercutio and Benvolio are outside in the dark, looking for Romeo so they can go home after the party. Little do they know that he is within earshot, wondering if he should approach Juliet's house, with whom he is now in love.

In this scene, Mercutio is more flippant about Romeo's being in love (he still thinks Romeo is in love with Rosaline). Perhaps this is because he doesn't know Romeo is listening, or perhaps he is a bit drunk from the party.

First, he mocks the state of being lovesick. He pretends to "conjure" Romeo, calling him up as he would ghost. He says he will know it's Romeo if Romeo appears "in the likeness of a sigh," or if he says, "Ay, me!" or makes a rhyme about love. We can hear his frustration with Romeo's constant moping.

Ghosts are usually conjured by the name of a certain person or entity, so Mercutio goes on to "conjure" Romeo by the name of Rosaline, and then by her various body parts, starting with her "bright eyes" and moving south until he becomes crude. When Benvolio tries to hush him, Mercutio's puns become cruder and cruder, particularly in lines 23 - 29 and 33 - 38.  

Romeo, overhearing this, remarks to himself, "He jests at scars that never felt a wound" (Act II, Scene 2, line 1).