Was there a social hierarchy in kilwa?

Yes, Kilwa was socially stratified.
The highest class was of the rulers who claimed direct Arab ancestry. They owned large plantations and controlled Indian Ocean trades.
The next was that of the Muslim-Swahili speakers who were mainly African. They were artisans and minor court officials.
The lowest class was of slaves who were non-Muslims. They worked on farms or estates or in production of cotton beads.