Any match with more than 18 points is considered OK. More the points, better the match.
Yes, Mangal Dosha matching is equally important. It is recommended that the level of Mangal Dosha in both the horoscopes much be almost equal.
In, Ashtakoota matching system, Nadi has been given highest point (8 points). If the total number of points more than 18, it is considered decent match even if it has Nadi Dosha.