I don't really view bounties as determined solely by strength or power so I think Katakuri's fits him just fine.
Having a billion berrie bounty and slightly above that was the hype Oda was going for, for both of them, with Katakuri getting additional hype as being undefeated and invincible in the Big Mom Pirates. It's just a means of creating hype, Katakuri being below King and Queen in bounty doesn’t mean he's weaker than them nor does it mean Jack's comparable to him in strength, it just means they're both notorious to a similar degree.
The Calamities, like their title states, are known to bring destruction wherever they go, Katakuri goes on missions for his family and such but I don't recall him and the Sweet Commanders needlessly causing destruction or wreeking havoc. Plus Kaido has a larger bounty than Big Mom and that's reflected on the bounties of their crew. Ultimately, plenty of factors go into bounties, not just strength. Katakuri's strength and portrayal are just fine in my opinion even if I don't view him as the strongest commander. He's got Conqueror's haki, signifying his will, awakening showing his devil fruit mastery, a special paramecia that was initially seemingly putting Luffy's fruit to shame and Observation haki as his strongest asset that's still a relevant power even now.
I'd say the dude's fine, a 1 billion bounty is high regardless and Jack having slightly below Katakuri just shows his hype, capacity for destruction and ruthlessness.