Thanks!
I quickly configured the texture compression and used a common “default” setting (the possibility to have a unique configuration for each type of asset is cool though).
It seems to work as intended for all elements (assets contained in atlases appear more or less blurry, depending on their atlas size etc.) except… the tilemap source image.
I even entered the path of the image itself (3rd line in path settings) but… still nothing 
Did I miss something? is there something specific to know about tilemap source image compression?