This is not necessarily the carpenter's fault. For budget reasons a theater company may use existing wall flats or platforms which have simply been used, painted, abused, and repainted too many times. It's impossible to build clean, straight, smooth walls with damaged, warped, lumpy flats. Even with newly built flats, a low budget may require using thinner plywood or fewer supports, or leave you scrounging for just one more (bent) screw. And sometimes, of course, the often-volunteer carpenters just aren't particularly skilled.
However it happens, the set designer has to cope.
Practical advice?
When wall flats are lumpiest, try to find or afford fabric to stretch over as "wallpaper." It's amazing what flaws this can disguise. Almost as effective is using tape judiciously at bad seams, then painting walls with texture - either real, like drywall mud, or faux, like painted sponge effects, spatter, scumbling, or faux marble etc. Texture hides lumps. Avoid gloss finishes and light colors, as these showcase mistakes; flat and dark colors hide them.
Here's a little detail to show how glaringly obvious sloppy construction can be. This shows fabric on a flat... badly installed. That top edge simply isn't pulled tight enough.
I spent a couple hours yesterday helping to stretch painted fabric on un-smooth flats - with a wonky stapler too! - and I can swear to you that it should and can look neater than that. Turned out great, actually.
