How strong would the blast yield be? Probably not very. Rocket explosions usually consist of an initial small blast that blows the fuel in one direction and the oxidizer in the other. The fuel ignites, but since it's not mixed with the oxidizer, it burns at the interface between the fuel and the surrounding air, and doesn't generate much of a shock wave.
How strong could it be? A worst-case scenario would probably be something such as an overpressure incident causing the fuel and oxidizer tanks to rupture on the ground before launch, giving them time to mix before igniting. Based on the numbers in the current Wikipedia article, complete combustion of the fuel in the Super Heavy would release around 39 TJ of energy, or roughly equivalent to 9 kilotons of TNT.