Question: Imagine a standard $^{235}\text{U}$ pellets scaled to the scale of Earth, I wonder, would it explode or just meltdown? I believe despite overshooting the critical mass, the heat produced is still too small to start sustained chain reaction? Right?

Answer: 
A critical mass of U-235 fuel is only 17 centimeters in diameter and weighs 52 kilograms. A sphere of U-235 the size of the earth would be impossible to assemble because it would go critical and explode long before you could get it that big.

The heat generated at criticality is not what sustains the chain reaction, it is the neutron flux released by the fissioning uranium atoms.
It is conceivable that Jupiter might have a radioactive center the size of a planetoid or planet. The high specific gravity of uranium and thorium suggests they might co-locate towards the center of a planet. The possibility of going critical seems more than just an academic question.

While uranium and thorium compounds (like oxides) have a lower density, one might expect these compounds to be chemically reduced in planetary interiors. Perhaps the Earth might have experienced, or is currently experiencing, fission in its interior.