It's hard to have an honest "like" response here. The upvote is so valuable, and crafting a quick comment that doesn't sound like it was spoken in Troll is pretty tough.
I think those "nice" comments are people trying to emulate that honest "like" response.
I know that I have had trouble with language here on simple comments to try to avoid sounding like I was an auto-comment troll.
That everything has a potential economic consequence directly attached to it does muddle things a bit as far as intentions go. I'd often like to just drop a "this was really nice" as opposed to "this was nice" but either will probably be dismissed all the same.