I like how thematic the card design is, and I think that it is not overpowered strictly from a comparison with the other cards that exist in the environment. It is on the powerful end of the spectrum, but not so much so that it warrants restriction in my opinion.
All that said, I tend to agree with mdc273 that I am not particularly fond of the card design, nor do I think it's particularly healthy for the game. It may sound like I'm overstating the impact of this card, and I don't mean to (which is why I wouldn't restrict it), but this card shares two characteristics with many cards that are restricted.
First, the triggered effect in my opinion reduces player choices, and thus reduces interactivity. There are a lot of cards that reduce player interaction/decision-making (rather than increase it), and it is only when the effect is particularly powerful that it becomes a problem (for example Burning in the Sand, Threat from the East, and Bear Island). I'm not sure whether it's fair to single this card out to pick on, given a lot of other cards similarly reduce options and decision-making (The Laughing Storm is the best example), but from a design standpoint, I think this effect misses the mark. At best, Margaery gives the controlling player some control options and toolboxing, but the decisions the controller makes about which characters to kneel aren't all that difficult. And there is some dangerous combo potential with this, for example by giving Margaery deadly, that could create NPE moments.
Second, there are no real trade-offs required by the controller, further reducing decision trees. What separates "good enough" design from "great design" is the creation of an effect that has a variety of uses, but which involve a difficult trade-off by the player. These opportunity costs are built into A Game of Thrones. When I kneel my character for a challenge, I choose a challenge type, and my character will be unable to participate in other challenges throughout the round. Similarly, when I choose a plot deck, I must balance gold, claim, plot effect and (usually less important) initiative. The problem with Margaery is that she does everything pretty well, a bit like Fury plots, Meera, Long Lances and many other restricted cards.
I suspect mdc273's reaction to this card has to do with the second point I made. Frankly, in a world where card effects look more like Margaery and less like real decisions, the game devolves into who puts the best cards in their deck rather than who makes the right decisions during gameplay.
I guess where I net out is that I'm OK with champion cards seeing a bit more play than the average card. And for that reason, Margaery's usefulness as a toolbox is fine, I guess. But if you want to talk about good design, then we should have another conversation about Ser Barristan Selmy and/or Sellsword Deserter. Both have very powerful effects, but require the owners to make very real and significant trade-offs throughout the game.
I may never win a larger tournament and have the ability to design my own card, but if I did, it'd be just like one of those.