What you sign or don't sign in your Git repo doesn't matter because NPM doesn't publish from a Git repo. Signing commits is still useful for your contributors and downstream forks but it won't have any effect on the users who use your package via NPM.

I think NPM is fully to blame here. Packages that exceed a certain level of popularity should require signing/strong 2FA. They should implement more schemes that publishers can optionally enable, like requiring mandatory sign-off from more than 1 maintainer before the package is available to download.

Then on the package page it should say: "[Warning] Weak publishing protection" or "[Checkmark] This package requires sign-off from accountA and accountB to publish".