I love how Perfectheart is canonically only the way she is because Sweetheart interfered with the experiment that created her. She's only considered 'perfect' after SWH touches the glass she's contained in and alters her form; Perfectheart is essentially a physical manifestation of Sweetheart's long for perfection. She is what SWH longs to be and pretends to be, but SWH also hates her since Perfectheart is so genuinely good. SWH wanted to marry herself, not what is in her eyes a mimicry of her perfection. To her, she already is perfect, having convinced herself as much, so suggesting that Perfectheart is better than her would cut into her deepest insecurities and affect her greatly.

Sweetheart is her own enemy. She could've been happy with Rococo, but then she locked him in the walls because she thought she deserved better. She could've been happy with the sprout moles, but then she set out on a quest for love because she thought she needed a lover. She could've been happy with Spaceboy, but then she ripped out his heart because he disagreed with her and she refused to believe she could be wrong. She could've been happy with Mutantheart, but she dismissed her immediately as below her and unworthy of her love. She could've been happy with Roboheart, but she rushed Molly's code and they were convinced RBH was programmed to kill instead of love.

But she never would have been happy with Perfectheart because Perfectheart is both what she loves and loathes. What she wants more than anything else but knows she'll never reach. SWH is too self-centred, too unempathetic, too rude, too emotionally unstable, too unwilling to learn from her mistakes. SWH has (if any) very little morals.

Perfectheart is perfect because Sweetheart wants to be, and that's the reason why the latter rejects her