I want to preface this with the awareness that sometimes dreams/visions are a way of showing and preparing us for what lies ahead. It arms us so that we can handle the specific situation it seems to present in representation. Other times, they are our inner most thoughts coming forward, getting the attention it needs because we don't want to think it in our waking stages for various reasons:
My interpretation of your dream is that there is a fear within you as a parent that your son might have married someone unattractive on inside. You say that she is a nice looking woman in reality, which causes me to think you are hopeful she is nice looking (beautiful) on the inside.
Time will tell as to whether this dream is your subconscious talking to you or if it's your intuition picking up on something about her personality.
In any case, it seems as though you should use the dream as a message to try to get to know her a little better (if you haven't already done so). Also, (if you haven't done this either), let your son know you are there for him.
As I type this to you I am feeling that in 1-2 years the truth will emerge.