You don't say something is "kick-assy", you say something is "kick-ass."
So I think the correct form would be "kick-assness".
You wouldn't use this in any formal setting, though.
I agree with awesomeness as an alternative.
english.stackexchange.com/users/1606/Claudiu
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.stackexchange.com/users/1475/zwol
I'd say "kick-assery" is a lot better than kickassiness since "kick-ass" stems from a verb.
english.stackexchange.com/users/677/Epaga
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.
english.stackexchange.com/users/1625/Albert Herring
Although it's rarely used in slang, it's neither an accepted nor a proper word.
Instead, I would use "awesomeness".
english.stackexchange.com/users/127/Mehper C. Palavuzlar
Even Wordnik turns up zilch, and Google shows a measly 300 something hits.
There doesn’t really seem to be a need for this word.
Futhermore, I agree with @Claudiu.
“ kickassness ” seems to be the term you’re looking for.
english.stackexchange.com/users/1637/Konrad Rudolph
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?
Is kickassiness an accepted word?
english.stackexchange.com
<vocabulary><grammaticality><slang>
english.stackexchange.com/questions/3874
english.stackexchange.com/users/397/Chandam
It is generally agreed that "kickassiness" is not an accepted or proper word in English. Other more acceptable alternatives could be "kickassness" or "awesomeness". The interpretation is highly subjective. Some people believe "kick-ass" is the correct adjective and not "kick-assy", therefore by deduction, "kick-assness" would be the correct from. Others believe that "kickassiness" is using similar reformed words such as "sarcasticness" and based on this logic would use "kickassness" or "kickassitude".  Another proposl is "kick-assery" as "kick-ass" stems from a verb. As English tends to be quite a flexible language, people would understand "kickassiness". Some suggest that it is rarely used in slang, so is not an accepted or proper word and should never be used formally. Neither do Google searches deliver a lot of hits, and therefore there is no real need for the word. "Awesomeness" is proposed as an alternative.
It is generally agreed that "kickassiness" is not an accepted or proper word in English. Other more acceptable alternatives could be "kickassness" or "awesomeness". This is neither an accepted nor a proper word. Instead, I would use "awesomeness" as an alternative. "Kick-assery" is a lot better than kickassiness since "kick-ass" stems from a verb or “ kickassness ”.
This is not a proper word. Alternative suggestions include "kick-assness" or "kickassness". However, "awesomeness" could also be used. It is generally agreed that "kickassiness" is not an accepted word in English. Other more acceptable alternatives could be "kickassness" or "awesomeness".
Use "kick-assy". Replace with "awesomeness" "Kickassiness" follows the formatting similar to other words,  "kick-assery" would be a better form as it stems from a verb. English language is flexible and "kickassiness" would be acceptable It is not a word to be used in formal settings. "Awesomeness" would be a better alternative. There are very few internet hits, there is no need for this word.
"Kick-assery" is a lot better than kickassiness since "kick-ass" stems from a verb or “ kickassness ”. Instead, I would use "awesomeness" as an alternative. This is neither an accepted nor a proper word.
"Kickassiness" is not an accepted word in English. A more acceptable word might be "kissassness". "Awesomeness" could be a more acceptable word. "Kick-assery" could be a more appropriate word.