system
Given the following post, respond as if you're commenting on Reddit:
human
[Batman 89] Was Joker Batman’s first real supervillian?

Had Batman only dealt with low level criminals at that point?
gpt
Most likely. He's still an urban legend at that point, with just a few terrified thugs speaking of him. The police and all journalists save for Alex Knox and Vicky Vale don't believe in him. Vale and Knox have basically nothing to go on (no mention of any major criminals he's faced in their investigation). It's considered something of a big deal that Commissioner Gordon has actually opened a file on the case. Even major figures in the criminal underground are shocked to discover he's real, like Jack Napier; crooks keep their ears to the ground regarding threats to their operations, if any big shots got taken down by the Bat they'd probably have heard about it.

And Batman deliberately encourages criminals to tell their friends about him, so it's not like he's trying to keep his existence secret. He does confiscate Vicky's evidence of him, though, so most likely he wants to keep the rumors on "invincible bat monster" and away from "guy in costume". By the end of the movie he's done enough that no one denies his existence any more, and the police start making use of the Bat Signal. 

So while it's not impossible that he fought a supervillain before Joker, it seems unlikely, because A) the strong implication is that it's very early in his career, so it would have needed to happen right out of the gate, and B) supervillain fights tend to be pretty large scale affairs, and it would have needed to happen entirely outside the view of the police, press, and underworld.