The hat mod is one of the very few that doesn't add actually blocks or functionality so it wouldn't ruin the game for anyone but as stated it's still a mod, requiring a modded client.
Relying on big projects like Forge and Bukkit is somewhat safe but to everything else I'd say no.
This isn't a Valve game, if the staff has to work on something I'd prefer that something to be improving the server and minimizing players drawbacks, not hats.