Everything is important.
Someone who is really tzanua, though, will also have good middos since she will not brag and try to get attention, and will be humble. It says “v’es tznuim chachma” in mishlei, she will be wise and “v’hatzneia leches im Elokecha” in Micha, she will also be a yarei shamayim. However you cannot tell that all from clothes, there are modest people who do not dress appropriately!
Similarly, someone who is really kind, will not want to cause others to sin and will dress appropriately. However we all know people who have a heart of gold and struggle in this area.
You need to look at the whole person. Everyone has strengths and weaknesses, you need to know your own and what you need in order to have a successful marriage.
Personally I think the most important midda in a marriage partner is commitment and the determination to make it work, and a certain measure of selflessness.