If Yamato was stronger than King, Kaido would have ordered the Calamities to look for him, not the Flying 6.
Black Maria saying it could be "challlenging" indicates that Yamato is most likely above them, individually, but not enough that they shouldn't be able to restrain her.
Ulti got hit because she was distracted. We saw against the gust sword from Nami that Ulti is actually quite fucking fast and can dodge attacks if she knows they are coming. Using that to say "Yamato one shots Flying 6" is, to me, quite erroneous.
There's also the consideration that Oda does a shitty job at powerscaling characters. Sometimes he uses bounties as an indication of strenght, sometimes he doesn't. Most of the times the bounties should indicate power ranks only inside the respective organizations (meaning Katakuri's 1B bounty doesn't mean he's below Queen's 1.3B, but Queen's 1.3B indicates he's above Jack's 1B. Know what I mean?). She once clashed equally against Ace and Ace was, at the time of his execution, a 550M bounty pirate. We don't actually know how much stronger Yamato got at this period of time. So I won't go saying things like "Yamato bodies all Beast Pirates aside from Kaido".
I currently have her around Jack's level. Now, if one thinks this is a good or a bad thing, well....you do you. I, at least, think that's a very good thing because I think Jack is hype. Even Kaido acknowledges Jack and protected him from the Sulong Dukes. No "jack, you are useless". Just a genuine sense of care.