O'Dim should be a legendary card though. He is very likely the most powerful being in the witcher world and well..... probably the devil. You can't get much more unique than he is. Being rare certainly doesn't do him justice. (and he is considered to be the best written character of Witcher 3 by most polls I have seen)
Who cares about how rare it is when the card itself is so weak. Like you said O'dimm is the strongest character in the witcher universe. So he should have (one of) the strongest card in Gwent. Regis card isn't that strong either. I really don't understand why some of the weaker characters (lore-wise) have stronger cards then some of the strongest characters in the witcher universe.