Thanks for the comments. Based on them I found https://github.com/snapcrafters/gimp/issues/21 which suggested copying the font to ~/.fonts/ (which didn’t work) and to /usr/local/share/fonts/ which fixed it - Epiphany can now see the font, and I can now set it as default in Firefox (tho Firefox is ignoring it in a page’s CSS).
It may mean the user doesn’t think their use is similar enough to the people who make the distro/DE, or trust the distro makers’ decision making ability.
If a distros’ makers think snaps are a good idea, or that the distro shouldn’t by default show available security updates, or have a UI that hides how many open instances there are of a program unless you hover over an icon, or hides the titles of those open programs, or hides panels; then the way I use a PC is too different from the way they do - and there are likely more things in the background that we disagree with which can’t as easily be changed like UI settings.