varying gap at the top with some trim board. It might have to be scribed to match the ceiling contours and then cut to match the scribed line with a coping saw or jig saw. Tack it to the front of the cabinets so it’s level, caulk any gaps with paintable caulk and paint it the ceiling color and it should blend in very nicely.
The width of the trim board will depend on how much variation you have in your ceiling and how much clearance you have between the cabinet doors and the ceiling. It could be 1/2 inch or 4 inches. The important thing is that the cabinet lower edge HAS to be level and even. It’s right at eye level and any variation are going to be VERY noticeable. Patches and variations up at the ceiling are a lot less noticeable.
If the gap is less than 1/2 inch then just filling it with foam backer rod and then smoothly caulking it with paintable caulk would work. After it dries paint it either the ceiling color or the cabinet color depending on what you think will look best.
There are too many variations to offer help on general statements so if you have some special situation post back with more information and maybe we can come up with other ways to hide the ripples in the ceilings.
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