The trick is not to show up at all. Most pointless meetings have too many participants. Meetings with fewer than 5 people tend to actually matter. Not 100%, but that's the trend. So the majority of pointless meetings you just...don't attend. It's incredibly rare for anyone to ask where you were, and if they do, have an excuse. "Outlook must not have popped up the notification for me", "I had a conflict", etc.

If outside of that you suspect it's going to be pointless, dial in. That way even if you can't just walk out, you can do something else.