Ridiculousness is a noun: the quality of being ridiculous — how silly or unreasonable something is. 'the ridiculousness of the rule'. Speakers often prefer a lighter shape, such as how ridiculous that is or the ridiculous part. Ridiculousness is a bit heavy; people use it when they want to name the quality itself, sometimes with a dry smile. It is correct, just not the everyday choice. Do not use it where you need an adverb (ridiculously) or a simple adjective (ridiculous).