why is rear wheel top tilted in after a lower ball joint changed?

Maybe you need a 4 wheel alignment.

You'll need an alignment for camber. Most alignments (the ~$70 kind) are just for toe-in or toe-out. You need the more expensive one sadly. If the place you had the ball joint done at doesn't do alignments (and most won't) ask if they have a relationship with any alignment shops in town and can pass along a discount.

some people put the ball joints on the wrong side. Have seen it done. Maybe need an alignment.