There's a difference between censoring and curating.

Popular subreddits don't seem to to curate for meaningful discussion. So jokes, memes, and other low-effort comments often make discussion on reddit close to worthless.

The exceptions are always either small niche subreddits where a common deep interest focuses the discussion, or they're curated subreddits e.g. the neutral politics subreddit.

And as for censoring, moderators are often accused of selectively enforcing rules to favour their agendas etc. I must admit I'm not sure how much that actually happens though.