This would be an understandable complaint if Cadance had been in the role for more than, like, a couple years.
(I do have the timescale correct here, right?)
I don't think it's even that long. I'd guess two years at most, probably less.
I mean, you'd think the pseudo-goddess of Love would have had an inkling that people don't just deeply hate you for no reason.
I'm sure she knows. That doesn't help tell
what reason. Sunset is rather unwilling to communicate with her or anyone, sees any interaction in terms of masks and tests, and even if you drugged her to be perfectly calm, and enspelled her to only tell the full truth as she understands, probably couldn't articulate the reason because she isn't really aware of it herself. The reason is pretty simple: Sunsets absentee mom brought home a new daughter, and she's the new favorite child. But you can bet it'll take Sunset a whole heap of character development before she can formulate that.
And ultimately, there's nothing Cadence could do. She could be the greatest mage since Star Swirl, and Sunset would still hate her, though her justification would change. Maybe she'd hate Cadence a little less, or maybe more, or herself more, but that doesn't really matter. The point is that Cadence got what Sunset had been working so hard for. Notice that Sunset never even considers that maybe Cadence does deserve it, that she
is better than Sunset, more deserving*. The closest Sunset gets to that is when on learning the conditions for ascension she grudgingly admits that Cadence must have done something impressive with her talent, but there's a pretty clear note that she was still just lucky and didn't
really deserve it. There's really nothing Cadence could've done, because Sunset's issue wasn't truly with Cadence in the first place.
Like, say that the Crystal Empire was around, and Cadence got wisked off there, and Celestia only occasionally visits. You bet that Sunset would not hate her anywhere near as much. She'd probably not be nice (she's not really nice to anyone except Celestia sometimes, and there's still a personality cult), but she also wouldn't be the pony that stole her mother, and also demonstrated that Sunset's way of getting her mom to be her mom was not working.
*Of course, the notion of being better or more deserving of love is a whole nother can. Let's not open that here.