I believe that it is illegal to place anything to obscure visibility or distract the driver so - assuming that they are in the windscreed then the answer is yes.
No they AREN'T! I got pulled over many a time with my HUGE fluffy dice over my rear view mirror and no one blinked an eye at them! And I had a nodding Cartman from South Park on my parcel shelf, a squidgy Cartmen on my dash and 4 dolls hanging in my windows! Not one person ever complained or arrested me for it!
Not being arrested for doing something does not mean that it is not ilegal - it just is not worthwhile for a policeman to arrest and prosecute when they have better things to do. If you had an accident and an assessor took a look at your car and saw dice, dolls, stickers etc then the insurance company may decide to decline you claim on the basis of your lack of visibility and as it is illegal (as far as I know) then you would not have much of a leg to stand on if it went further.
And who says I didn't have an accident in that time?!! Ok, you're right. I hate being wrong. But my car was decorated and I wasn't arrested purely because what is the point in taking it through the courts? Too expensive for the police and to be honest you're right it isn't worth their while! So if you want fluffy dice the moral of the story is to remove the dice straight after an accident!
My children bought fluffy dice for my car and we hang them anywhere but the rearview mirror. When I drive with the children in the car I tend to drive more cautiously and hate to have my vision obscured. Check out this link.