I have been looking for a word that means kickassiness, but haven't come across one. Is this an accepted word? If not, what word can be used in its place?
Kickassiness uses the often-sarcastic -iness ending (compare "truthiness", "maverickiness", and the phonetically similar "helpy"); so, analogously to those words, it gives the impression that something is trying to pose as kickass but is not succeeding. I'd probably go with "kickassness" or "kickassitude" if that was not the desired impression.
All the above are perfectly cromulent productions from the base word; don't let any prescriptivist tell you different.
English - natural language in general, is "productive", which is to say that it happily accommodates new words derived from existing ones on standard patterns. Slang and familiar language are by nature more productive than formal language. Thus, if you use "kickassiness", people (at least, people who understand "kick-ass") will mostly understand you even if they have not heard it before. Whether or not they adopt it themselves is in the lap of the gods, of course (and the answers by @Epago and @Claudiu suggest that it is unlikely because slightly more natural alternatives already have some currency). It is also unlikely to impress English teachers, judges in courts of law, or prospective employers, of course, but you probably already knew that.