Tangled up in Ella Enchanted's hot mess of wealth and power and social class is a major emphasis on rules and order. This works on two levels: the pressure for people to act according to the rules of their social standing, and the pressure on Ella to follow rules and commands due to her curse. You don't have to have a Ph.D. to see a connection here: forcing individuals to conform to society's rules is the very basis of society. Sure, some of the rules are good—like, don't randomly murder people. Some? Not so much. As Ella fights to disobey the strictures placed upon her personally, she's also fighting stupid and arbitrary social norms about how to properly curtsy and which fork to use for which meat. Feel free to disagree with us here… but don't expect to come to our dinner parties.
In Ella Enchanted, rules are arbitrary and meaningless.
Ella's rebellious behavior is pure spite. It does not serve a useful purpose.