The problem is that the redemption is incredibly forced and rushed. White has very little characterization and her reasoning and motives for being Space Hitler are never discussed or even acknowledged.

A major part really is due to how little screentime there is of White, but there is some issue in general with how Gems are portrayed. We don't see much of Homeworld except the negative, and in a lot of areas the writing doesn't make sense.

It's implied that Blue's colonies were hurting because she was so depressed, but it's never brought up again. We never see any other planet for that matter, seeing what it's like under Homeworld rule. Neither does it help that they are full on genocidal and eugenics focused, making it impossible to see the ones responsible fully redeemed.

Imagine if Hitler didn't die. He was instead captured. Whilst in custody, he waxes poetic about how sorry he is and how he really wants to do what's best for his people. Now imagine if after that the Allied Powers just shrugged their shoulders and let him go, just assuming he really did change his ways.

Hell no. Even if by some massive stroke of stupidity that really did just change everything. That doesn't change all of the horrific things Hitler is responsible for. Being sorry doesn't absolve you of punishment. Which is a common issue with modern media, where a character does something bad but the writing/characters ignore it for one reason or another.

Its not just "modern media". Its been happening for years in literally every form of media on the planet. Stroheim is a literal Nazi in jojo part 2 and he ends up an ally of the main characters, Vegeta blew up planets, etc etc. Redemption arcs that feel wishy washy aren't due to the writer thinking their actions are easily redeemed, they are usually done for utilitarian reasons like having an interesting character dynamic or because the execs wanted it.

White diamonds redemption, personally, felt off, I agree. I think what they were trying to go for was that she was an emotionless machine, and that Steven managed to basically break her emotionlessness. That's why they put so much emphasis on her getting embarrassed. They didn't have enough time to establish her as a character properly however, so it came off awfully