Lol funny you'd jump to that conclusion. That makes no sense. If that were the case, Mari wouldn't be given a script at all. Maybe there's a reason Anno left Mari without criticism and let her be natural. Her character certainly embodies a really strong natural child archetype. I don't know how you got "let her act it her own way" to "let her do whatever she wants."
The only layers we've gotten from Mari are that she's really enthusiastic about being social, which is shown by her movements and her uninhibited parts of her actions, and that she also knows more, and that more is the something viewer did not know, in a very serious manner. So basically she's a free child, but balanced in the other egos as well. This is how she's marketed in almost every one of her scenes because it's her personality and her egogram. It almost appears as if she's normal and just well balanced, but tagging along for the ride(again, the representation of a free child: no responsibility but excited to be around). I think that's what makes her strange in the series though; she's just normal.
Whereas Asuka represents a strong critical parent ego, and Shinji the adapted child one, Mari takes the third one of the series. I think her placement is fine, to be honest. If there's a trip scene in FINAL, I really hope we're shown a happy-go-lucky Mari perspective.