How do I repair the bumpy area where the wall meets the ceiling?

Any chance it may be just the cutting in paint job that's off and not the ceiling/walls? You can install crown molding in the corners.

You can use a simple wallpaper border.

Some people install mouldings, some do murals, but if you want it absolutely flat, you'd need to rent scaffolding, then sand down the uneven surfaces and repaint.

In the interim, you could prime well and repaint the lower walls in a lighter wall color to minimize the distinction.

Thanks, all! This might sound really odd but has anyone seen that L shaped rubber (I assume it’s rubber) that they say you can use where the tub meets the floor instead of caulking? Anyone think that could work on the wall?

You can create a new straight "visual edge" by lowering the ceiling paint down the wall about 3/8 to 1/2 of an inch.

Use a thin straightedge long enough to span those irregular sags and dips. ( A 4' or 5' section of trim molding could work assuming it is somewhat rigid and won't flex too much).

Lightly place the straight edge along the ceiling/wall corner (don't push it into position so hard that it flexes...you want it to bridge across those waves).

Strike a faint pencil line along the length of it.

Add painter's tape following just below the pencil line.

Using the ceiling color paint, fill in the small gap between ceiling and tape.

Remove the tape and you're done.

If you're doing this from a ladder, all the steps can be completed in one position if you have a helper to hand things up and down.