Re: "It will make her happy and is just a rug"--
I think the problem is that often it doesn't stop with one rug.
If Daughter In Law says "Oooh, thank you, how kind of you to give us a rug" out of politeness, MIL may get the message that DIL loves being bought things for the house, and will keep on getting more and more stuff.
As time goes on, trying to say something about this ("Actually, I'd like to choose my own things for the house") is going to get more and more difficult to say, because you've spent years faking joy whenever you're given a potted plant or towel rail or picture or toilet mat or garden decoration.
It's easy to ignore this kind of stuff if you have a big house with lots of space, but some of us have small flats or terraces. Being given household items we don't want is annoying, because we don't have all these extra rooms and lofts and cupboards and therefore can't just shove them out of the way, and now have a choice of either never being able to buy things that we ourselves like, or putting up with an overstuffed cluttered house/flat, or having to throw things away and put them into landfill (which feels horrible).
I just think it's better to explain kindly "It's so lovely of you to think of us like this but I'm afraid we tend to buy things like this ourselves, so if you buy us household items, it will just duplicate our stuff. In future, please always phone and check with me if you are thinking about buying a household item for us so that neither of us ends up wasting our money [and when you do phone, I will definitely say, "No, don't buy it"] In the meantime, could you take this with you and put it in your house instead?"