I think questions of the "This one specific Hidden Service isn't loading" variety would be off-topic either way. These are often temporary or unstable, and trying to diagnose every single "site down" case would be impractical.
In a more general sense, our job is not to judge content or actions. Tor protects users from government restrictions of free speech, and "illegal/unsolicited" usage would vary from country to country.
That said, I think there's a level of common sense that "How do I hide my child porn?" or "Can Tor help me kill my spouse?" questions would be flagged and closed. If enough of the community agrees I'll go ahead and add that to What topics can I ask about here? in the Help Center and as an Off-Topic Close Reason.