What episode did the line appear in, and what was the context?
Also, are you sure Homer didn’t say, “Kiss my curvy butt goodbye”? That was my first thought in reading the OP, and a bit of googling for “kiss my Kirby butt goodbye” turned up MANY hits for “Then kiss my curvy butt goodbye” (in “Homer goes to college”, said as Homer exited said college). I didn’t scroll down far very through the hits, but didn’t see any for “Kirby butt”. “Curvy butt”, of course, would simply be an acknowledgment of the largeness and roundness of his bottom.
If he did say “Kirby butt”, a possibility that occurs to me is that it is a reference to the videogame character “Kirby”, who, IIRC, acquired powers by consuming his enemies. Saying he has a “Kirby butt” could be an obscure way of acknowledging his voracious appetite. But that seems like quite a stretch.