The quick answer? Nope, you're definitely not too dumb to score 720+ on the GMAT. And the GMAT doesn't really have much to do with how dumb or smart you are, anyway... but it's undeniably a spectacular measure of how well you fill in little bubbles under tremendous time pressure.
The truth is that it's really, really hard to improve from a 550 to a 660 to a 720, but it definitely does happen. Here, check out one of my all-time favorite GMAT Club stories:
420-570-590-700-harvard-business-school-132221.html. He didn't quite get to a 720, but he did get a 700 and an admit to HBS... and he's an Indian male! My favorite part is when that very same guy went on an
MGMAT forum, and some (perhaps fake?) admissions consultant told him that he had no prayer of getting into HBS. Classic.
So sure, some people naturally score a 700 without trying too hard, but those people are freaks, and you don't need to talk to them right now.
The bottom line is that you can absolutely collect another 40+ points if you put in enough work. Honestly, I find it encouraging that your RC is strong; that's a sign that you aren't inherently an imprecise reader, and you're probably capable of bumping up your CR and SC skills. SC is particularly "learnable": if you can become absolutely bulletproof at recognizing the most commonly tested SC issues (pronouns, parallelism, modifier placement, comparisons, etc.), you'll be fine. And if your reading is strong enough to be a killer on RC, you can ultimately get to the same level on CR. Keep practicing with official materials for CR and RC (OG, verbal supplement, GMATPrep Question Pack, LSAT if you need some extra questions), and you'll get there.
Good luck, and don't let this stupid test get you down!