~/.config/mimeapps.list contains a line “terminal=foot.desktop” (tried also without .desktop).
I don't think that is a real option.
There is no standard way to set the default terminal emulator, you need to tell your launcher application(sometimes through your DE settings) to use that terminal.
For example, j4-dmenu-desktop has the option --term.
As a file managers I use lf and nnn, they both contain .desktop-files but I can’t launch them with keybinds or menu launcher. Same applies to vim.desktop, nothing happens.
How are you launching these programs? For keyboard shortcuts you generally need to specifically run the terminal emulator together with the program: bindsym Mod4+Return exec alacritty -e hollywood
Edit. I managed to find a workaround for lf and nnn by editing the Exec= line in /usr/share/applications/*.desktop file. (Exec=/usr/bin/foot -e nnn) but I still can’t figure the swayimg imageviewer.
I would advise you to copy those files to ~/.local/share/applications so they do not get overwritten during updates.