I thnk that the narrator - or Austen if you like is saying that Mary would problaby be a reasonably OK wife to Edmund... like most women she'd shake down and give way to him to an extent... and that she'd at least publcily support him, even if they might soemtimes disagree over issues or grumble in private. I think it woudl depend on thier financial situation.. if they had enogh to live on comfortably nad she could get away to London for a bit of time each year, she'd be all right and owuld make the best of it, if she loved him....but it woudl be a second best marraige becauase she would be straining against his wishes for a completely rural way of life etc.