Why is it ironic that Phoebe asks Holden not to “swear so much” in The Catcher in the Rye?

You are refering to Chapter 22 in which Holden speaks to
Phoebe having gone into his parents' home and waking her up to talk to her. I suppose
the irony of this quote from Phoebe when she tells her brother off for swearing so much
lies in the fact that Holden has spent the entire novel up until this stage engaged in
swearing repeatedly. Clearly, Phoebe is unaware of this, but we know as readers that
Holden is not somebody who feels that there is any problem with swearing, and so the
irony lies in the way that Phoebe tells her brother to do something that he is clearly
incapable of doing or that he obviously sees no reason to do. Of course, the irony is
shown when in a matter of a few paragraphs Holden is back to swearing with his customary
panache, though he does choose to swear more in his own internal thoughts than openly so
that Phoebe can hear him.