I recommend against adding Mastodon for reasons below.
Irrelevant nitpick
Another personal reason for me is that mastodon is not similar to any other popular social media, and exposure to some of the more popular servers will radicalize some folks against mastodon itself given how insufferable people can be and how much toxicity abounds in the usage of in-group terms (before grumbling, the same is true for all other social media, but the smaller user base makes it seem a bigger issue).
Social media handles are a prominent public identity thing, and allowing others to host it will cede control over your public persona to another person and/or entities that may target the host infrastructure itself. Might be relevant to note this for the users, will matter in certain threat models especially for famous personalities.
Not completely. Providers comply with local laws, which seem to be getting tighter everywhere. Providers in states with less free speech might also be forced to censor content or refuse to allow federation to certain servers.