You don't need the Comesa yellow card for Ethiopia. Supposedly you need Ethiopian mandatory 3rd party insurance, but likely nobody will ever check. If you continue to other countries, it makes sense to get a Comesa card. It extends your Ethiopian insurance to cover other african countries. This is usually cheaper than buying new insurance as you go. Comesa is the African equivalent of the european green card.
Shop around, I paid 250 Birr for a full year coverage of all african Comesa countries except Egypt, Sudan and DRC, which I had already been to.