Is It Ok To Install Tiles Directly On Drywall Kitchen New Construction, I Want To Install Slate Subway Tiles As A Backsplash Over Painted Drywall. Ok Or ?

Kitchen new construction, I want to install slate subway tiles as a backsplash over painted drywall. ok or ? - is it ok to install tiles directly on drywall

I need to install Backer first card or can I install it directly on painted drywall and prepare for an apartment? The contractor left the electric fields and recepticals to 1 / 4 inches in the preparation of slate. Should I use a mortar or thin?