I made a buttercream cake with fondant accent pieces for tomorrow can I leave the cake out over night because of the fondant or should I refridgerate the cake

Unless you have a perishable filling in the cake, it will be fine at room temp.

I agree too. It should be fine, however, I always put mine in the fridge, especially stacked ones, with and without fondant. this firms them up and makes traveling easier. I

I wouldn't refrigerate if it were me. I did refrigerate a bc cake once that had fondant decorations and the condensation caused the color from the fondant to bleed onto the buttercream.