I am not a Julia Roberts fan at all, in fact I think she is extremely overrated and borders on talentless...
However, she is a huge celebrity and extremely recognizable. Thus, I can understand why she would want her group to have their own boat on Jungle Cruise. It's not unreasonable to assume that there's a chance that her safety and privacy could be comprimised in a boat full of strangers.
On the other hand, the JC ride operators aren't obligated by any rules to fulfil her request of a private boat. I'm sure they could have done so out of courtesy, but there was no obligation. She should realize that by spending a day in DL, during the middle of summer no less, she is completely forfeiting her privacy.
Whether or not she was being a diva depends on whether she was polite while she made this request. I don't think the mere request was diva-ish, because it seems pretty reasonable given her fame.