Why was lord Muu's fissioned clone not affected by the Earth Release: Ultra-Added-Weight Rock Technique?
Mū splitted first (using the Fission Technique) before Ōnoki made him (his fission clone) heavier.  

This was the order:


Naruto attacked Mū using Planetary Rasengan
Mū splitted his body while being hit by Naruto's Planetary Rasengan
Somehow no one noticed this
Ōnoki came to his fission clone and made him heavier