I've been watching some other Jokers play recently and kind of diving in with a whole new mentality after a month-long hiatus and there have been a few things that I've noticed people are not doing. Maybe this stuff is common knowledge and there's a good reason why I don't see it used, but I'll throw it out anyway:
1) D3 is godlike. Now I'm pretty sure we know it and I see other people use it, but I don't see people follow it up as well as they could. D3 is of course a hard knockdown, meaning no invincible wakeups. A well spaced D3 also allows for a retardedly ambiguous ji2 which I never see anyone use. It stuffs all wakeups due to the hard knockdown on D3 and it's so ambiguous that I always do ji2>3>2 then wait to cancel because I rarely know which side it will hit on.
Now I know, D3 is unsafe on block, so despite the range it can be bad if they see it coming. Well... yes, but it is a great midrange tool, and when mixed in properly, is godlike. This leads me to our other godlike midrange option...
2) D2. I don't see people use this enough as a spacing tool, and more importantly, cancel it into teeth or flower to make it safe. D2 has one of the best horizontal ranges of any AA move (Yeah fuck you Aquaman) and more disjointed horizontal range than most normals in this game. Now admittedly, it is horribly unsafe on block, BUT it cancels ridiculously fast, to the point where I think it might actually be safe even when canceled into teeth (Would need accurate frame data to know for sure though).
Perhaps the most important reason to put D2 into your midrange game though is because of the benefits it gives. Assuming you canceled into teeth, it gives you the following:
-If it anti-airs, you get a free teeth combo.
-If it hits them on the ground, you have a free dash/jump in and have teeth in place for pressure.
-On block it forces them to do something because they are now standing on teeth, allowing you to react to what they do.
Oh and it also allows us to stop relying so much on jump attacks to get in, which makes us easy to read and AA.
3) Use teeth to control space, using it to shut down other people's options. This one came to mind when watching SteveOSupremo play Grrrrr the other day (Sorry to put you on blast here. You have a good Joker and I enjoyed watching that set.). By keeping teeth in front of him constantly then just waited for Bane to do something, you shut out all of his mid/full screen approach options, most notably the charge. Now I know it has armor, but the thing is that if you block and have teeth in front of you, the teeth hit after the armor disappears. Another thing to note is that Bane's charge is not fast enough for him to use it to punish you for pulling teeth unless you throw teeth after he starts the charge.
That's just one example, but this basic premise applies to many matchups. By keeping teeth on the ground, we do limit our opponent's options.
-Close teeth prevent Scorpion from doing a MB teleport (He walks forward and hits the teeth, trading if the TP hit and being full combo punished if you blocked.).
-Close teeth prevent characters with front/back teleports from being able to teleport in front of you, making it easier to read and punish them.
-Any character who tries to approach you has to go around them, making them more predictable, and when combined with D2 cancels, can make it very hard to get in on Joker at mid range.
-Close teeth punish BA if he divekicks (although I do realize that's not the biggest problem in that MU and that trying to pull teeth will probably result in lightning, jazz hands or black Magic, but I'm just sayin'.).
-Slides and charge moves (Except Shazam's Psycho Crusher) in general all get punished hard, regardless of how positive they may be on block (Looking at you KF!) if you block them with teeth in front of you. The same applies to a lot of jump in attacks.
You get the point.
TL DR version: Joker may not be a good character, but to be honest I don't feel we're playing him quite right either. Most of us seem to be playing a super aggressive close range game, and while Joker is great at that when he has pressure on his opponent, I feel we're all neglecting our mid range game, which is where we shine when in neutral positions.