It looks like the mcomix package is not available in bionic (yet?). https://packages.ubuntu.com/search?keywords=mcomix&searchon=names&suite=all&section=all
It is available on debian stable (currently stretch), albeit only because debian stable naturally lags behind ubuntu. I suggest you give debian a try if you haven't already! https://packages.debian.org/source/stretch/mcomix
You could also download an mcomix archive directly from its sourceforge project page and have a look at the project dependencies, usually detailed in some installation instructions / readme file, and verify if your current distribution satisfies these dependencies, and compile it from there.
https://sourceforge.net/projects/mcomix/files/
The negative aspect of this approach, which does not seem to be critical in this case, is that you'll have to download and compile again every time you want to update to a more recent version.