Is there some particular reason that you're here?"..."Not this again.""Not what again?"said Clary."Every time I annoy him, he retreats into his No Mundanes Allowed tree house."Simon pointed at Jace.

Simon was telling Clary that whenever he does something that upsets Jace, Jace retreats into his own world and isolates himself. He likely won’t talk to Simon or anyone else that he perceives as “mundane”. By saying “not this again”, he is expressing his frustration that this is something he has seen Jace do before.