it depends. makeup can make your skin look better sometimes, because it acts as a barrier from the environment. however, obviously not putting makeup on your skin means not coating it with unnecessary gunk can help as well.
i love makeup. I'm not going to swear off it, but I really don't wear it usually. I'm trying to get my skin to be perfect. (probably not going to happen) its pretty good right now and most people say I don't need foundation anyway (which doesn't actually matter to me, but its nice to hear).
i don't know that you will necessarily improve the condition of your skin, though. not everyone has skin issues, not all makeup leads to skin issues, etc. its more likely that if you take care of your skin and have an appropriate regimen for YOUR skin, and wear makeup that suits your skin as well, eat a balanced diet and consume things that are thought to help skin, drink lots of water, and are gentle in removing makeup, that you will have good skin and improve the condition of it. if you use products that don't work well with your skin, whether its cleanser, makeup, moisturizer, scrub, etc. you will experience some negative affect on your skin. makeup is no different from anything else.
its like putting styling product in your hair everyday. not using the styling product is not going to automatically improve your hair. depending on the quality of the product you're using (if its some amazing great-for-hair styling product), it may do the opposite. you can eat a good meal, exercise, take hair vitamins, condition and moisturize as necessary, and still use that styling product and it may or may not negatively affect you.
however, it is much easier to fix your skin without adding on makeup. that's a given. if your makeup is high quality or medicated, it could have positive effects on your skin though. in any case, if you're not doing all the other things you need to do to improve your skin, unless your makeup is some specifically horrifying formula or just not a good brand or formula for your face or you sleep with makeup on, it won't make much of a difference if you stop wearing it.