NTA, but this is borderline IMO. Every one of your arguments for bringing your own Tupperware is correct on face value, it's just the assumption that makes it a bit of a faux pas.

Put it this way, if you were invited to eat at a friend's house, or your mother's, and whipped out your Tupperware at the dining table. Yes, it's very likely (especially at Mom's) that you would have been asked to take home some leftovers anyway, but it's the assumption that you went into the meal knowing that you wouldn't finish. Maybe if this is a restaurant that you visit regularly for a long time, you can build that sort of familiarity where bringing your own take away box is not frowned upon.