There are a lot of people who just reply "Nice post", and other things, without providing any feedback. I suspect they may just be looking to add to their post count & reputation count, by commenting anything, even just a picture without addressing the content of the posts. This is a bit annoying. I think I may start to flag them as you suggest. It adds no value whatsoever to the post or Steemit... I hope they don't get part of the curation rewards for commenting such things... lol.
Peace.
Nice post, duh, I mean what a great reply, I.E. I totally agree with you.
I always just check their page for their previous comments first. Their overall comments give a much better idea of whether or not it's a bot