I disagree that they've been giving him a god-like feel since the Gatewatch has been a thing. Before that, he did tend to have a kind of incredibly powerful mage quality as I touched on in my first post. But I don't think that's the case now. I read him as more the brains/support from most of the cards now.

As for being a player stand in vs Mary Sue, that isn't necessarily the same thing. If they did play him off as an awesome, god-like planeswalker to try to get people into him, then he would fall into both categories. But it's more accurate to say that they made the character into what they assume players will identify with. A nerdy guy who isn't physically strong, great at what he's supposed to do (being the Guildpact for him), or even all that good with the ladies. But he is very intelligent. And given they've consistently said that Jace is the most popular planeswalker, I'd say that has worked. I think they used to beef up his power/importance to help push them, but I don't believe that's the case since the shift they've done to the story.