*Yes, it’s possible that Frank and Nancy simply didn’t tell her that Puddles was supposed to be on a diet. But since Nancy (whose idea the whole diet thing strikes me as) has always been portrayed as competent and organized, while Luann has always been shown to be scatterbrained, oblivious and lacking in self-discipline, I’m gonna go ahead and assume the fault doesn’t lie with the parents here.
“Luann is an incompetent dog owner*, part 734”…
*Yes, it’s possible that Frank and Nancy simply didn’t tell her that Puddles was supposed to be on a diet. But since Nancy (whose idea the whole diet thing strikes me as) has always been portrayed as competent and organized, while Luann has always been shown to be scatterbrained, oblivious and lacking in self-discipline, I’m gonna go ahead and assume the fault doesn’t lie with the parents here.