If the largest distributions with the most users such as Ubuntu, Fedora, openSUSE, Debian etc. use systemd, then yes, I guess we can consider systemd “mainstream”.
No AppImageKit components require systemd though, at the moment.
AppImages may run on more niche systems, especially if they come with the usual libraries as part of their base system, but we don’t make any such guarantees. Here is a list of what we assume to be present in every target system: