Hokusai can't really refund farm to the extent that Surfdred can, but she really isn't intended to.

Like you said, the base NP gen is less than half of Surfdred's, but overkill is a 1.5x increase in NP gen, so she won't be going from 100 back to 100 from the NP alone like Surf does.

However, the way she works is that she stacks defense down debuffs by virtue of her third skill and has a really good Arts card. Put a star generation buff like Cas Gil and an Arts buff and she starts looping with NPXA crits while also generating stars due to her silly hitcounts. And after each loop, her NP and facecards get stronger due to the def debuffs.

Add some Poster Girl shenanigans so she gets access to her Arts cards and she flat out enables 3 turn clears of CQs.