I'm just gonna move my thoughts here on Joker vs Flash.
Joker - Outfoosies flash with mostly D2. Can catch dashes with D1 buffered into crowbar (although kinda difficult). Teeth can be used to temporarily halt movement. These are all benefits on Joker's side.
All the flash needs is 1 opportunity to kill our entire lifebar, and that opportunity can come pretty easily. For example, I mention earlier that D2 outfoosies Flash, However, it's punishable and, even at max range, it gives flash a free jump in into a 50/50. D2 low canister can be jumped out of a full combo punished. D2 teeth is probably the best option at max range (or else it'll be punished with b22 or 112 combo). D2 crowbar is our safest option, which is great, but it's -8 (somewhere around there) and likewise gives flash the opportunity to get his game started up.
Once the Flash is in, he's gonna be there for a while. Running man stance pressure is of course useful against all chars, so I'm not going to simply say it's especially bad for joker. However, I will say that, even if we're lucky to get a D1, the best we can get is D1 crowbar or D1 212 teeth for pressure. On knockdown, Flash can just bully us for days considering we don't have a wakeup. Delayed wakeup can be blown up by a neutral jump 50/50, and backdash can be chased down by F2 or stuffed by D1D2 (the D1D2 has happened to me before, but I haven't tested that in the lab yet with different situations. I will comment again once I test that out).
Let's not forget Flash can shut down most of Joker's offense with D2. Joker has great jump ins, and that's easily anti-aired with Flash's D2. So we're reduced to the ground game....... I already explained the situation with D2. Dash into B1 shouldn't work because Joker has no direct overheads (unless you're talking about our 25 frame B2). Nothing else should work on the highest level.
I hope I've explained the MU well. If you disagree or like to add, please reply!