I firmly believe she is the best atom-weight women's fighter in the world (she did lose the belt in her last Invicta fight, but I believe she knew she was UFC bound and it affected her performance,) but this isn't atom-weight. This is Straw-weight. She's best at 105, but at 115 she's probably borderline top 5 or top 3. It's possible for her to win the belt, but it's hard to know where she really stands until she faces a top 5 fighter at straw-weight. It might be that you're correct. She might have the right combination of striking skills and grappling to dominate the division, but I think she beats everybody but the top 3, personally.
She's a proper Straw-weight, and her size will become a disadvantage the second she's fighting the more highly skilled opponents like Joanna who has three inches of height and reach. That's already a decent advantage between two average(ish) sized males, but for smaller women, that's a significant difference Waterson's fast, but Joanna is fast, too. Waterson's legs are fairly good and she's good at using them to control distance, so who knows? But it's not as cut and dry as 'Waterson destroys everybody, easily.'
That being said, I'm a huge fan of Waterson, I'm a huge fan of Joanna. I'd love to see them fight.