GNOME Font Viewer is a simple app to display the fonts installed on your system.
It however has a habit of crashing - especially for the first few times of usage.
font-manager is a new GTK app that is quick and allows you to manage your fonts.
It is undergoing a rewrite to GTK4 but with an unknown timescale. We have asked a question about libadwaita. There are a few issues outstanding but seems to be robust.
For the moment we should consider changing to font-manager