Fire Fox is higher than Luigi's SJP?
Nahhhhhhhhh.
SJP is not as bad as people make it out to be tbh.
As an OoS option, this move is solid. Most dash attacks can be punished this way, and then you die at 70. Lots of people may say top players don't randomly dash attack to shields. Ok, that's good. But the threat of FJP means you cannot dash attack to Luigi's shield, ever. At best you get grabbed. At worst you're dying at 70, and that's without rage on most.
'Dashing' type moves are probably MORE susceptible to this. Falcon Kick and Wizard Foot for example, carry Luigi along with them if the late hit hits. Other moves include Bayo's Side-B, I think. Most of these moves don't really see much use for top level of plays, but is still something to note.
But then people hit your shield and you slide too far... ok, nice attack m8. I'm just gonna use something other than Up-B and see if it works instead. With safe smashes I don't exactly need to kill at 70 with super high risk move.
Punishing moves tend to be trickier. Moves' FAFs tend to not allow you to hug your opponent and land a frame 8 attack. At most, you can punish people who recover high with this, but if you miss... eh.
As a recovery option, this move aligns well with Luigi's recovery. Get to far below and position with Cyclone, and snap to the ledge with this. It goes higher with momentum, so as long as you are still in your double jump 'rising' part, you'll still get a height boost, and does no one notice how ****ing high Luigi's double jump is? Do you really waht him to go any higher? Nah, I'd take slight horizontal movement from Brawl first.
The juicy part, you can set this move up. Unspaced jab 1 or 2 can lead to this move, albeit the opponent can just shield and hard punish you, so the risk is very high. Still an option nonetheless.
Soft N-air to FJP works on fast fallers. It has 10% range for Falcon where it kills and true combos iirc. I've seen Sheik and Fox being the victim of this too (The former was done out of a reverse U-air btw). Typically done out of jab locks, but it's possible out of D-air spike at specific percents.
My point is, this move is not meant to be a main kill move. If Luigi only had this to kill with, sure, this move is garbage. But in conjuction with his kit, this move works more than decently enough. This move deals a hard punish that can kill well below 70 with rage, the reward is insanely high. Obviously you're not going to land this move very much (Else you's be a star in Salty Spitoon lol). This move was, IMO, meant as a secondary kill option. Your main kill tools are your smashes, but if the opponent mess up badly such as dash attacking to your shield or suffer from a high landing lag... that's when FJP comes in, to punish people for making such horrible mistakes that can be capitalized harder.
In a vaccuum, this move is pretty trash, but in gameplay, you'll land this move a lot more times than you'd expect tbh. Mr. CC had landed a lot of this in the past (And he still does) and it wasn't before I realized SJP's true purpose that I started to land a lot more of this. You don't use it to fish for kills, you use it as a harder, riskier punish option.
When you consider the reward, the risk and difficulty of landing it become much more fair tbh.
TL;DR: Don't use FJP to fish for kills, but use it merely as 'another' option to punish that gives off much more reward and risk.