Here is a reply from Samsung, the last paragraph would explain your higher than normal idle temps.
Removing the motherboard heatsink may give you better temps.
Excellent idea, we'd all be interested in your results.
Hi. Very interesting. So what are your peak temps during heavy use?
The sticker on an m.2 drive is meant to be thermally conductive. So you did the right thing leaving it on.
As for thermal pads and heatsinks, they're generally fine. Drives are intended to work without them but if you have particularly stifling conditions they can help. The interesting thing is that the way I understand it the NAND chips actually prefer to be hotter and the controller chip performs optimally when cooled. It's just that expecting the users to know the difference and designing heat sinks that cool the controllers on hundreds of different SSD models really isn't practical.