While I don't use Ubuntu myself, several of my friends do, and to my greatest surprise, it seems like that MComix is not available in the official repositories. However, Comix, which came to an end in 2009, is.
What is the reason for that? MComix is clearly superior to Comix, it is actively developed, free of any major bugs, stable etc. It doesn't make much sense to me to have a dead project available for the users, and a much better alternative not.
Thanks for the compliments. I believe it is in Ubuntu's package repository, but only in the yet-to-be-released version 12.10 (http://packages.ubuntu.com/quantal/mcomix). Credits for this go to a kind person who created and maintains a Debian package of MComix, which was recently transfered into Ubuntu's sytem. Why no stand-alone package was created I cannot say. Like you, I do not use Ubuntu myself and am not too familiar with the packaging system, else I might have done the packaging myself.
In the mean time, some unofficial repositories have pre-made packages, such as https://launchpad.net/~blca or https://launchpad.net/~createsc/+archive/3beol.
FYI: According to http://packages.ubuntu.com and http://www.debian.org/distrib/packages, respectively, MComix is currently available in