If you think you can get the national merit, yes. If not, no, it isn’t really necessary, but it can’t hurt.
But don’t feel like studying for this would be a waste of time. It would indirectly (and directly) be studying for the ACT as well. Enhancing your math skills and reading comprehension, regardless of the test, is going to help your standardized test scores for sure. Definitely don’t kill yourself over it, but don’t do what I did either (not study at all because you assume you won’t get it and then end up a few points away from what’s projected to be the national merit cutoff for you state)…ah well, at least I’ll likely get a commended.
Also, don’t feel weighed down by math. Since I took calc last year, all the past math kinda blurs together for me, but I think the majority of standardized testing tests algebra and geometry…not sure how much of that algebra is algebra 1 vs. algebra 2, but precalc is very little of the tests (some conics and matrices for ACT, but I believe that’s it). Focus on filling some of those gaps of knowledge, increase your reading comprehension, and you’ll be golden! Good luck 