Those are listed as "Some common reasons to flag" and typically "I don't like this" could fall under "Disagreement on rewards", as you had mentioned.

If someone thinks the post doesn't deserve rewards at all, then they can flag it as it's a disagreement on rewards. It doesn't require it to be low quality or low effort for someone to disagree on rewards.

But still, those are only listed as some common reasons to flag, as people can flag for any reason. Just like they can upvote without having a specific reason to upvote content.