Mel was telling a story about Christ. He obviously had to include dialog and story elements that are not present in the Bible.
Do you take a similar attitude towards the movie Ten Commandments and other movies about bible stories? If so, it would make making a movie or writing a story about any biblical character quite impossible. Applying the same principle to history in general would make all historical fiction or history-based movies impossible.
I don’t think Mel ever claimed his movie is what happened. He told a story based on the Bible and to the best of his ability not contradicting it, but obviously expanding on many story elements.
Personally, I see nothing wrong with this.
License in the matter of the message constructed by God to inform His own about him is not a good thing. I have a few ideas with respect to a short film about God in Heaven. There is an element of humor in the execution but it is meant to portray God and his message as I perceive Him to be. Problem: I have no way of knowing if any thing in it might be offensive to God. Or rather I worry that I am not well informed enough to know for sure, or that too much of it is my own opinion. I get a hell of a laugh refining it and turning it over in my mind but I doubt I'll ever do anything with it for the reasons stated. I consider what the evil spin artist of these latter days could do with such license and I shudder.