A lot of Jirachis get given really weird/ strange nicknames, probably more than most other Pokemon on GTS. Is there some kinda joke to do with Jirachi and how did the whole Jirachi nickname thing start?
Most of the Jirachi's up on GTS are probably hacked.
My guess is that some jerks hack in Jirachi's and Nickname them 'F--- You'and and other crazy stuff.
It isn't just Jirachi either, Celebi is quite a Victim sometimes as well
Look here is you want to see in more-depth
Well the only common one ive really seen is Sleepy which is most likely a tribute to Youtuber Sleepy Jirachi
Other then that Its Just the randomness of people choosing nicknames.