The last time we saw Mandy succumb to that most base of all emotions--love--was in the similarly one-word-titled episode "Crushed," when she fell for Billy's goth schoolmate Piff. At the end, she declared that love is for the weak.
"Heartburn" has a set-up that's more puzzling. Grim's camera shows what your true heart is and it shows that half of Irwin's is identical to the whole of Mandy's--pure, unadulterated evil. Mandy losing her sense of self, fearing that Irwin (of all people) may be her ultimate soulmate, becomes a non-sequitur, reaching a pinnacle as she lies on the couch in tears, trying to shake off the notion she's falling for the nebbish. I suspect that the majority of fans that were turned off by this episode were Billy-Mandy "shippers," repulsed that Mandy would allow Irwin to kiss her. As I see it, Mandy betrays herself, what she stands for, on behalf of the show's staff experimenting haphazardly with chromosones X and Y. There should certainly have been a keen episode of Mandy's inhibitions let loose, but "Heartburn" wasn't it.