Why would it not be? You can form "open-mindedness" from "open minded" the same you can form "yellowness" from "yellow" or "coldness" from "cold". It's a valid

Why would it not be? You can form "open-mindedness" from "open minded" the same you can form "yellowness" from "yellow" or "coldness" from "cold". It's a valid way to make a word from almost any adjective. Syntactically it's weird because unlike most other Germanic languages English likes to put spaces into compound words. But that hasn't stopped open-mindedness from catching on and getting mentioned in dictionaries.