Why is the rate of nitration greater for thiophene than toluene?
I would suggest two reasons:


Thiophene is an electron-rich compound because of presence of two lone pairs on the sulphur atom. Hence, it attracts electrophiles readily.  
Though its aromaticity is lesser than that of benzene, the delocalisation is among five atoms in thiophene whereas in benzene it is among six atoms. Hence electron density in thiophene is more.