what is the weather like in late may (E.g 24 May) or early may (E.g 1 May) in kunming?

According to the past weather records, the average temperature in May is about 19C and the highest temperature can be about 28C. In May, it will start the rainy season, so you may meet rainy days from time to time.
Generally, I suggest you take along both short-sleeved and long-sleeved shirts. A light coat may help at cool nights, especially in rainy days.