Not sure where the cut off is or should be, but it doesn't feel standard to me.

On my anecdotal desktop install there are two packages that use cmake; awesome and taskwarrior.

Grep'ing the ebuilds on this Gentoo box, shows me ~400 packages out of ~16000 that use cmake(excluding KDE where it clearly is standard). A lot of the remaining ones seem to be very project specific too(70 leechcraft, 11 fcitx, 10 opensync).