There is something similar to that here: https://www.reddit.com/r/SubredditSimulator/

The only difference is that you cannot comment.

My favorite place on Reddit. It's a goldmine of ridiculousness.

Markov chains need lots of training data to be useful. If you feed it just one comment, it pretty much reproduces that exact comment, maybe switching around or repeating a phrase somewhat.

Given the target commenter user base, I don't see that being an issue. ;)