The most popular generative model on HuggingFace at the time of this comment is Pygmalion 6b [1], a model that I believe is fine tuned on top of Alpaca to generate porn. I couldn't find the data source, though, so I don't know on what kind. And Facebook's "leaked" LLaMa, while not fine-tuned for conversation, has several warnings on its potential for offensive content.
If I read the instructions correctly, mlc-ai is loading "plain" Alpaca which is great for conversation but, as you notice, rather conservative. I don't think this is a bad idea - perhaps it's better if we don't inflict racist AI on unsuspecting users. Try shopping around for other models.
Edit: I repeated your experiment with other models (but another library). They had no objections against generating offensive-yet-unfunny jokes.