In season 2, Diaspro was complaining to Flora how she's spent most of her life learning etiquette and how marrying a prince was the whole point.
In season 3, she used a love potion to win Sky over. Like she didn't care if Sky really returned her feelings or not.
In season 6, she actually helped to try and kill Bloom. And I really REALLY doubt that Sky would've wanted to marry her if she did succeed.
I wonder if Diaspro actually loves Sky, or if she is just using him to become future queen of Eracklyon. Now in season 6, Diaspro seemed more interested in getting revenge at Bloom than actually getting Sky. Like, perhaps she actually is over Sky, but wants Bloom to feel how she felt when Sky broke off the engagement in season 1. Or maybe Diaspro wants Sky because he was the only prince that put up with her rude acts.