I think this would be quite good as it would also set the tone of the post to a positive one; HN can be quite negative or critical (and I am guilty of this myself on multiple occasions, especially recently) and having the Gratitude flag may go some way to keeping these posts having a positive focus. I enjoy debate and we need it for a healthy society but it can also be draining and I think it would be nice to have a little corner of HN that you can go to that’s a bit cheerier for those times when you just want to share in some positivity.
I suspect it would be an attractive nuisance for spam and marketing in practice.
Even if that were managed, I am not sure how intellectually interesting it would be.
Finally, I suspect that it would attract about as much cynicism, snark, skepticism, and negativity as anyplace else on HN that allows comments.
Of course I might be wrong and YMMV. Good luck.
(In general, it's usually better to start smaller as a test, rather than committing to a permanent, bigger thing.)
Could you please let me know which app are you talking about here?
We do have regular threads on Ask HN for things like this, and I think they do the trick.
Your sentiment is noble, but actions speak louder than words.