This product (Japanese language) is claimed to cool down your smartphone, etc, battery by two degrees centigrade - see the photo a little way down the page.
It claims to be a graphene-based material, and looking on the web I see lots of articles about how graphene is an excellent heat conductor, and would work well for CPU cooling fins.
However, reading the small print, it suggests sticking it directly on top of your battery, underneath the cover, if you have one - this would appear to negate any cooling effect as there is nowhere for the hot air to escape to, and furthermore outside the case, your hand is going to be on top of the sticker, so if anything it would transfer heat from your hand to the device!
Thanko has released a sticker they call the “Mobile Heat Sink” for cooling down a heated smartphone, PC battery, tablet, game device etc. Just by attaching it on a device, it can make it cool down by about -2 C degrees.
However, it isn't clear that this English site has any direct knowledge of the product.
Overall, I get the impression that this is a variant on the EMR-eating sticker scam. Is my skepticism justified?