In fact, I would argue that Joker has the advantage on Loki mentally based simply on the fact that, while the God of Mischief’s greatest weakness is his superiority inferiority complex, the Joker has no such flaw for he is way too damaged already to fall to the mercy of his ego.
Now, you may think that Loki has the upper hand on a psychologically disturbed maniac in that realm, but if you recall, the Joker is the same man who tricked Superman into killing his own wife, so I would not underestimate this clown’s brain power.
Just as many a battle against humans have ended for him, Loki would be too distracted by his own hubris to predict (or even comprehend) how a mere mortal could defeat him, but Joker always has a plan (Joker Venom mixed with steroids to increase his strength or a quick read-up at Sanctum Santorum’s library) to ensure he has the laugh.