What makes you think that the Tobi Roppo are all Supernova level characters ?
Each and every member of the Mid/Weak Trio has show that it has what it takes to fight very strong opponents :
Franky's very durable and has two excellent attacks (his laser and air cannon) even outside of the Franky Shogun - which considerably boost his resilience.
Robin's able to deal with super-fast characters and can create giant limbs, it should be powerful enough to hurt even Ancient Zoans users.
Brook is fast and can freeze peoples, and has shown after the timeskip that his swordmanship has improved; he was fast enough to surprise Big Mom at least twice : when he hurt Prometheus and when he sliced Zeus.
Chopper tanked a direct hit from Crazy Big Mom without looking worse to wear and while he hasn't has the chance to test the mettle of his Monster Point yet, he's still a good fighter. Plus, having him facing powerful Zoans is basically a must in this arc ! How else will he learn more about his DF otherwise ?
Nami's fragile but she has some of the most powerful attacks out there now, with Zeus, her Weatheria stuff and the one "magic wand" she still hasn't used yet from the TS.
Usopp took a direct hit from a 4t bat in the head back in Alabasta. He trained for two years, unlocked one form of Haki, has a wide range of Green Pop at his disposal, and his Skull Explosion are able to take out fighting fishes in one hit, just like the attacks of Caesar a 300 millions berries logia.
And it's not my fault that, so far, the only headliner who has shown to be worth something is Hawkins, a member of the Worst Generation.
Sure, the Tobi Roppo are the elite Headliners. But considering the level of their playmates, it isn't saying much so far...