However, checking the dictionary, "overweight" can be a noun meaning "weight in excess". All that "overweightness" would be doing is changing an adjective to a noun, and that's apparently not required. Thus, "Prevalence of Overweight and Obesity" is acceptable usage.

What is the noun form of overweight?

"Overweight" can be a noun, but its use is rather limited. People may suffer from being overweight, where "overweight" is an adjective.

What's a nice way to say overweight?

In this page you can discover 30 synonyms, antonyms, idiomatic expressions, and related words for overweight, like: corpulent, plump, chubby, tubby, obese, ample, huge, fat, hefty, heavy and pudgy.