I don't think it would have flopped, but Joker lent a lot to it... Ledger's performance is iconic, so that definitely helped, and the Ledger hype made huge deal, in my town, at least... Some people were like "Batman's kinda for kids, but I'm a HUGE fan of Heath Ledger..." And the girls at my school were like "I hate Batman, but Heath Ledger is SO cute..." So Ledger's presence really increased the ticket sales, imo...
But I think it would have done slightly better than Begins had Joker been for some reason absent (wouldn't have happened, though)...
No I think one of the reasons that Batman 1989 and the Dark Knight are the two most successful movies of the Batman franchises is because everyone wants to see Batman versus the Joker. That is the most epic battle in the comic book industry in my opinion and it seems that it always will be. I think no matter how many times the Batman series may go through reboots even in the future the biggest Batman movies will always be the ones where he has to deal with the Joker because I think the Joker is Batman's most popular villain, and a popular villain period in his own right. People love to see him.