Silly would be a mistake, but you can still make them fun, and you can give Batman a sense of humour. I think people have some diluted memories of the Burton Batman movies; they were actually full of little quips and funny bits, and Keaton had a great way of doing deadpan humour as Batman and be a little more goofy as Bruce Wayne. He actually was pretty close to Tony Stark, looking back at it.
Think of this Batman with a team. That’d be JLA Batman. Deadpanning all the time, the guy where the others are never sure if he’s being funny or serious with the things he’s saying. It’d be great.
And it wouldn’t even be all that hard. Couldn’t do it with Affleck though, he doesn’t have that kind of charm.