Chad Kroeger RIPS Avril Lavigne's Ex-Hubby -- You're a NOBODY!!!

Avril Lavigne's ex-husband ... JUST GOT OWNED BY THE GUY FROM NICKELBACK!!!

Here's what happened -- Deryck Whibley and his hot GF Ari Cooper went to a Halloween party this weekend dressed up as Avril and her new fiance Chad Kroeger.

Deryck was Avril ... Ari was Chad ... it's sorta funny, but kinda weird ... and a photo of the cross-dressing couple made it to Twitter (see above).

Now for the good part -- Chad Kroeger saw the pic and responded on his Twitter page with the following message: