I just posted this in a private DJI Mavic Mini facebook page, just copy/pasted here.

"I see a lot of people putting dark wraps/stickers on their Mini. I hope you know that Mini doesn't have any cooling system inside, just airflow going trough when flying. My point is that in the summer time when temperatures are much higher and the sun beating on the top of the drone can potentially lead in overheating of your Mini as dark colours absorbe much more heat.
Just my 2c."

Many YouTube reviews have notice the Mini's light color (and size) against the sky.  Method to their maddest?

Hi. Thank you for creating this informative post. Yes, you are correct. When placing the sticker on the DJI Mavic Mini please avoid covering the vents or placing the sticker on the spot where the heat sink is placed to avoid any issues specially overheating. This may can affect how the heat dissipates on the said drone. Thank you for your valued support.

dji sells a creative sticker kit and it's not exactly light and it's not reflective.  in most of the free world, you'll be fine with attaching stickers to your Mavic Mini when properly applies it should be fine; it's not the delicate drone so many are making it out to be.

For what I've felt by touch the bottom part of the Mini is the one that gets hot while the top part remains fairly cool.  Dark colors can affect the overall temperature but I believe by a small margin.