The key difference between a yoyo and a car is that B-grade on a yoyo typically means aesthetic flaws, like ano not being quite up to par, or a bit of vibe, they typically play just as well on the string as their a grade counter parts.
I’ve had FG and B grade yoyos that I couldn’t tell any difference other than an engraving marking them as B-grade. This is especially true with b grade Gen-yo in my experience. Manufacturers typically won’t sell a yoyo with wobble or severe vibe as B-grade, never that I’ve heard of at least.
Basically if having flawless ano matters to you or dead smoothness then you don’t want b-grades. But those aren’t big deals to me as they don’t affect string play.