As long as they are still wrapped def do it although I do remember a story from when I was a student nurse. Little old lady every week used to gi e district nurse a bag of brazil nuts saying my son buys them I don't like them you might as well have them. DN ate them every week. One week met the son and thought she ought to tell him he was wasting his money - he replied but I buy chocolate brazil nuts!! Turns out old lady was sucking the chocolate off and giving the nuts to the nurse so lesson from an ex nurse be careful what food you accept from a patient!
I'll definitely say YANBU; I'm considering palming off a couple of items of clothing that my MIL bought for DD. Fortunately she didn't wrap them before she gave them to me and even more fortunately DH agrees that they are hideous
If I give food presents (usually to teachers etc.) I hope if they can't use them they will at least recycle them. After all its the thought that counts. Although how much thought there is in your case I'm not sure, but my in laws could give me the Ferro Rocher too, and I hate them too.
An entirely appropriate re-gifting. She (MIL) sounds a selfish cow, and it should give you an extra glow, to know that you have made her bitterly-given presents into good ones. Would she spit with rage, to have done good?!