Steven Universe - I don't want to see Jasper 'redeemed' this way

You have the right to express your opinion.

But I have the right to call your opinion stupid.

Indeed, she stayed an unlikable brute to the very end, while Peridot became a lovable dork and Lapis a deadpan snarker.
You seem to assume the redemption made them act out of character, but nothing could be further from the truth: In the case of Peridot, it only extended on a personality we already had glimpses of before: She freaked out over cartoon stickers, asked if Steven was the new species, grumbled about her things bing broken, grumbled about her mission on the ship, called everyone a clod during her fight with the crystal gems and acted like a stereotype hammy cartoon villain during friend-ship. She was a dork way before we took her limb-enhancers, she never was some kind of dark and cool villain. Her redemption only extend and added deepness to her personality.
Now, on to Lapis. I am going to sound mean, but I think it’s nonetheless truth: Lapis didn’t had much of a personality before she came to live in the barn. The few scenes we saw with her just told us two things: Lapis life suck and she is kind of a downer. That’s it, that was the extent of her character before we learned her backstory and saw her interactions with other peoples. Now, we know she is a flawed, kind of messed up gem but with a good heart inside, that is slowly healing from her trauma, learn to enjoy life and found herself liking to meep-morp and snarking.
Redemption doesn’t make villains boring if it is handled correctly, and it was the case with the Steven Universe.