As someone who has played The Joker since launch, my three biggest pieces of advice for playing a decent joker player are the following:
1) TECH ROLL (seriously). I'm always amazed at how many online players don't tech roll at all. Any good Joker player will tell you that tech rolling on knockdown essentially gets you out of the vast majority of his mid-screen teeth setups. (The big exception to this is his BGB teeth setup, which produces a hard knockdown, and thus, can not be tech-rolled and can only be escaped by a handful of wake-ups) Now, the corner is another issue entirely. As others have already noted, stay the hell out of the corner against The Joker.
2) Learn to consistently anti-air on reaction. This takes practice, but with Aquaman's ridiculously awesome d2 and normals in general, this shouldn't be too hard for you to get the hang of. The Joker arguably has the best air-normals in the game, but unfortunately, his lack of mobility and mediocre overall zoning force him to rely on jumping in far too often. Learn to Punish The Joker HARD for jumping in recklessly. It will hurt his overall effectiveness considerably against you.
3) Lastly, get comfortable blocking low up close and ducking past half screen. The Joker has no quick overhead launcher (This is why Joker players begged NRS for months and months to make his f2 an overhead, which would have drastically helped his competitive viability, IMO) and Gun (his only viable zoning tool) hits high.
Good Luck! (But not too much, because honestly, you won't need it after some practice with Aquaman, lol)
