At least as far as US law is concerned, a federally hosted and administrated social media platform gets interesting with America's unusually strong free speech laws, since there's content which is legal but unethical which they likely would not be allowed to block or moderate, such as bullying, hate speech, misinformation, etc. but also illegal content would be immediately moderated away, which might include content that falls into legal grey areas or ethical but technically illegal content, like someone copy/pasting the contents of a paywalled article, or discussing any kind of DRM or digital security bypass

Honestly I think there's good reason for governments to host a Mastodon instance for their representatives to use for communications, but inviting the public to use it might get weird for sure