Just because someone sucks, doesnt mean they arent a singer. Although rappers call themselves rappers so they dont actually claim to be singing...
I do both and I dont differentiate between the two. I go from rapping to singing within a song and feels the same to me.
Im not claiming that Ice Cube could sing opera (the highest form of singing...) but neither could John Lennon... So if you use the logic that only "good" singers are singers then someone whose standards are higher than yours will argue that Elvis isnt a singer... And there were many people who said that back in his day.
Many of the people who argue that rappers arent singers wouldnt hesitate to refer to Bob Dylan as a singer...
Open your eyes!