You're right, it does look pretty bad.
This appears to be some quirk when using the elementary icon theme with Geary.
It's possible to set the GTK_THEME to the Adwaita icon theme which looks much better.
You can set the theme when running Geary with the following Flatpak command.
flatpak run --env=GTK_THEME=Adwaita org.gnome.Geary
To avoid the hassle of ...