I think back-air is fine as a move as it has a lot low knock back and it prevents inkling from going for grabs or other face forward moves (like their ink bombs). It’s kinda like nerfing Captain Falcon’s backair when it prevents him from using his other options (like his up-special and grabs). It has more commitment than moves like Wolf’s or Fox’s neutral-air. If that move had a lot knockback like other back-airs, I would ask for fair nerfs (which they probably did when they made the move). What will discourage its uses are potential rewards. But these rewards are often absent. A move like Ike’s neutral-air is actually borderline broken as it actually has combos and kill-confirms, and it has way more coverages than Inkling’s back-air.
Inkling already has a lot of ink usage and ironically, it’s the thing that encourages the back-air usage the most. If that up-b is nerf because of the ink usage. Then Joker is going to receive more popularity and Inkling will probably be abandoned. No one likes a character with that fatel weakness when nearly everyone else has ridiculous up-b’s but with smashes, b-specials, and even jabs that aren’t tied to a ink tank. The real reason why ink management isn’t that noticeable as a weakness is because it’s mainly for limiting Inkling’s offensive options (including edge pressuring). But they make these ink-related moves very unlikely to kill (even the roller as it has too many ridiculous factors connecting to it), so it feels mostly pointless. So, Inkling’s trouble to kill is supposed to be its weakness when it runs out of ink. But, damage alone doesn’t kill an opponent and Roller is ridiculously badly designed. Heck, they can just make splatbomb a kill move as it drains 1/6 of ink, is pretty slow and punishable, and the edge is designed so stupid, that it doesn’t hit an opponent when it explodes right on them.
They can just readjust neutral-b as a move that wastes a lot more ink than before, yet holds an opponent in place for a smash attack if their percent is high enough (like about 120% or more). They can slightly nerf the verrtical distance of the up-b but lower down the lag from the down-air and slow down side-b’s fall, just to perserve their off-stage game.