Why is it ironic that Mercutio teases Romeo about Rosaline?

Mercutio first teases Romeo about Rosaline in act 2, scene 1, when Romeo has disappeared beyond the orchard wall after the Capulet party. Assuming that Romeo can hear him, Mercutio summons Romeo in the name of "Rosaline's bright eyes . . . high forehead . . . scarlet lip . . . and quivering thigh." Mercutio says that he summons Romeo only to "raise up him." This is a crude sexual innuendo. Mercutio is implying that Romeo is so besotted with Rosaline's beauty that he will become physically aroused simply at the mention of her various physical parts. There is a dramatic irony here because we, the audience, are aware at this point in the play, that Romeo is no longer in love with Rosaline, but is now in love with Juliet. Mercutio isn't aware of this.
In act 2, scene 4, Mercutio teases Romeo about Rosaline again. And again there is dramatic irony because Mercutio still does not know what we, the audience, do, namely that Romeo now loves Juliet. Mercutio teases Romeo by insinuating that he has been away all night having sex with Rosaline. He says to Romeo that "a case as yours constrains a man to bow in the hams." In other words, a distraction like Rosaline has, Mercutio implies, caused Romeo to flex his buttocks in the act of sexual intercourse.
The second irony to Mercutio's teasing of Romeo is that it perhaps doesn't make much difference whether Mercutio knows that Rosaline has been replaced by Juliet or not. Arguably, there is little difference between the two. Romeo falls recklessly and obsessively in love with both. He also expresses lustful desires for both. Perhaps, therefore, the only real difference between Romeo's professed love for Rosaline, and his professed love for Juliet, is that the latter reciprocated his feelings whereas the former did not. In this sense, it really doesn't matter very much that Mercutio's teasing is directed in part at the wrong girl.

The ever-mischievous Mercutio loves nothing more than to tease his good friend Romeo over his incorrigibly romantic nature. He's pretty cynical when it comes to matters of the heart and so finds Romeo's capacity for falling head over heels in love at the drop of a hat a source of endless amusement.
By the time we've reached Act II Scene IV, Romeo is on the verge of marrying his beloved Juliet. Yet Mercutio doesn't know anything about it. When Romeo doesn't show up for their meeting one night, he wrongly assumes that it's because he's still hopelessly in love with Rosaline:

Why, that same pale hard-hearted wench, that Rosaline,
Torments him so, that he will sure run mad.

But Mercutio doesn't know the half of it. Romeo's head over heels in love alright, but not with Rosaline. She was just an infatuation, but Juliet is the real thing. Mercutio's merciless teasing of Romeo is a fine example of dramatic irony, which is where the audience knows something that one or more of the characters does not.