I think if you look up MIL in the dictionary, you will find that one of the definitions is "person who gives unwanted gifts and/or junk." I think many of us can relate.
My suggestion is not to say anything since, the way things are, your MIL is happy, and you only have to deal with getting rid of the stuff. Hopefully you can at least donate it somewhere. On the other hand, if you say something, her feelings WILL be hurt - there is no way to sugarcoat it. Which is the worse outcome: having to dispose of things you don't want, or hurting your well-intentioned, albeit misguided, MIL? There's your answer.
If it really is intolerable for you to get these unwanted presents, what about gently suggesting to her:
(1) that she set up a college fund (or you set it up for her since she is overseas) for your son, exclusively from her, that she can fund, a little bit at a time, with the money that she is otherwise spending on gifts for all of you?
(2) if she has access to the internet, that she choose gifts for your son (to be shipped to your home) from a particular online store since you can't go wrong with their selection (e.g., Gymboree) and gifts for you and your husband, if she insists on giving things to you instead of just your son, where items are easily returnable even without a receipt (e.g., Wal-Mart, Kohl's)?
Good luck - it is an unpleasant situation.