Does successfully reporting a thread give you more activity?
No.
I try to report spam threads as much as possible,
That's a good thing to do.
but I'm wondering if it'll reflect on your activity score.
Unfortunately, no. :-(
I think it should, but I don't ever see a boost from it.
Because it will not.
Would it be possible to implement to incentivize more users to help out the moderators
No. I mean, what is the point? Mods voluntarily do their job, so you should, too.