I think the onboarding and new user experience for Mint could be better, but I think there’s one important thing that I think makes Mint a good intro distro: Its Ubuntu base.
If you look up guides for “linux” it usually gives instructions for Ubuntu, which usually also apply to Mint. Likewise, if you look for software downloads you tend to find Ubuntu debs.
I know flatpak fixes these issues to an extent, but I think we’re not there yet.