In principle, the exposed pad should have thermal vias. There are different techniques in use, e.g. small open vias, that fill with solder, tented vias, unfortunately reducing the effective pad area or plugged vias with galvanical copper surface.
All these variants are more or less meanings less for a two-layer board as you have. To reduce the thermal resistance, you should connect a sufficient large topside copper area to the pad (without thermal relief, of course).