If you enable Pre-multiplied Alpha in the spritesheet import settings it helps a little.

However, the source image is just too low-resolution to look good. Even though the texture is 4K, a lot of that space is wasted, you can try packing it better to get more resolution. Or lowering the framerate so you can increase the frame resolution. But there is only so much you can do with a low resolution texture, it's never going to look that good.