Is it obligatory to fast two consecutive months as expiation for unintentionally killing a person in a car accident, despite paying the blood money and serving a prison sentence?

Since you caused the killing and were sentenced to pay the blood money (diyya), expiation (kaffara) is incumbent upon you. This expiation is the freeing of a believing slave. If you do not find one or are unable to do so, then you must fast for two consecutive months. This is based on the Almighty’s saying: "And whoever kills a believer by mistake – then the freeing of a believing slave and a compensation (diyya) presented to his family, unless they remit it as charity." (Quran 4:92), up to His saying: "And he who finds none – then a fast for two consecutive months as repentance to Allah. And Allah is ever Knowing and Wise." (Quran 4:92).