Riot gave Jinx her old Q back so she could see pro play...and hardly saw pro play
Answer's really simple, honestly.
Jinx is a hyper carry with no real steroid when it matters. Twitch has an AD steroid on one button and an AS steroid on the other. Trist has her Q. Vayne has AD on R. Xayah has W. Kai'Sa has E. Kog'Maw has W. If Jinx wants to get into "overdrive", she has to destroy something. This is the crux of the issue.
Why would you want a steroid that only comes on when you're winning? It's like getting a massive, double value Soraka ult only after you ace the enemy. Yeah, you're hyper and you're cleaning up the fight, but most steroids are used to gain the advantage in fights, and then people rely on resets or team synergy to finish them. Jinx's steroid comes on when the most crucial part of the team fight is already finished. Whenever one or two people go down, most of the time the fight's decided at that point, and the rest is seeing if the remaining 3 or so champs can escape without conceding more gold. Sure, Jinx can more reliably chase down these targets, but if you don't win the team fight in the first place, this detail is meaningless.
But she has her Q. That's a steroid, right? Well, yes...but it's irrelevant. It's a steroid to her attack speed, but its 525 range makes it so
she's too easily exploitable to actually want to use it
the moment someone with a gap closer comes up. If she switches to rockets, other hyper carries can either get in her range and run her over, like Vayne, or they can (roughly) match her range and run her over because she has a massive attack speed disadvantage in comparison to other hyper carries when using rockets, like Kog'Maw or Caitlyn...or they can be Tristana or Twitch and do either of these things at their leisure. Either she gets out-ranged or she gets out-gunned in comparison to other hyper carries, mainly because they don't pay a price for using their own tools.
Well, back in season 5, this wasn't the case. Since then, Riot realized that Jinx's kit is just too much of everything. She had scaling, range, damage, aoe, all that jazz. Turning Runaan's into a crit item was good for her, but great for her competition, and the nerfs they put on her, between locking her stats behind random barriers and nerfing every button she has, made it so she sort of fell out of relevancy.
Well they gave her the Q back. They also removed -5 AD to do it, and we ALL SHOULD KNOW how crippling of a nerf that is for marksmen. They also didn't remove the AS penalty from her rockets, so she's still horrifically outgunned for actually using her rockets during fights in comparison to someone else that'll either force themselves into her space and maul her or just pepper her down with faster, deadlier autos from similar ranges (or both).
So just give her back 5 AD, right? Well...no. Jinx's design is horribly generic. It doesn't have any key weaknesses or strengths. It's just sorta...there, as far as the strategic side of the game goes. If it's strong, she can easily replace most or all hyper carries in the game for being a safer early game pick in comparison to other hyper carries while still scaling relatively as well as them. Riot's tried to sort of "create a defining strength" for the champion out of her existing tools, but these changes have mostly fallen flat (oh shit, now you can't dash over her E!).
...so how to fix? Ultimately, you don't. Jinx is great for the game as far as theme and story goes, but her kit's execution is from a time where Riot didn't think too hard when it came to "breadth vs. depth" in the game. Kind of like how Capcom used to create a bunch of Ryu-styled characters to add to their roster instead of making legitimately different characters, in a way. Jinx doesn't offer any big "thing" to balance her around, so if she's good enough to be a pick beyond solo queue, it's because she's the best marksman in the game...like in season 5. You'd have to rework some of her abilities and her passive to shift her to a clear cut direction as a hyper carry, but her execution on the thematic/story side of the game's so good as is, I don't think Riot's willing to do that.
It sucks, because Jinx is one of my favorite characters in the game. I'd be more interested in the LCS year round if champions I cared about actually showed up more, but they don't, and haven't for years. Well...Poppy does when she isn't horribly nerfed so that's good, but not much else.
Also saw no play.
was only played once and lost.
were played twice and both won one lost one.

who made up 34 of the remaining 44 bot lane picks... and only managed between them a 41% win rate.