As I just wrote, the world is a messy and complex place. As loathsome as Trump and what he stands for is, there are some things he is correct about. One of them is the funding of NATO.
It is true that other NATO countries do not spend as much per capita on their militaries as the United States does. Therefore it is also true that those countries are to some degree getting a free ride at our expense. That this is happening is a legitimate topic for public debate, one that has generally been absent from the national rhetoric.
Therefore some questioning of this state of affairs is in fact completely appropriate. What is not appropriate is to do so, as Trump is doing, in the name of facilitating an appeasement of Russia.