Flatpak already creates executable wrappers for all applications as part of regular installs, though they’re by default named as the full package name.
For when inkscape has been installed into the system-wide Flatpak installation, you could simply symlink it like; ln -s /var/lib/flatpak/exports/bin/org.inkscape.Inkscape /usr/local/bin/inkscape
For the user-local installation, the exported runnable is in ~/.local/share/flatpak/exports/bin instead.