Some over-generalisations here. The output during the ska years was already huge.
There were a good number of sophisticated vocals, though some of them were sung by teenagers who were still developping their talents.
Don't tell me that ska vocals by people like Lord Tanamo, Eric Morris, Roy Panton, Justin Hinds & The Dominoes, The Maytals, The Wailers, even more so The Gaylads, Delroy Wilson, The Techniques, Alton & The Flames, Slim Smith, Stranger & Ken, and a a lot of lesser-known artists were more second-rate than those in the later stages of JA music. Just like in the later years, there were many moods, influences and varying degrees of vocal prowesses.
After all, not all rocksteady sounds as smooth as Treasure Isle's, not all roots reggae sounds as fine as The wAILING Souls' or the Mighty Diamonds', for instance.
Besides it all, i don't think that many better vocalists often graced JA sides than Jackie Opel, who, though not being Jamaican, was a huge vocal talent.
Of course, if you opt for grating vocals, like those of Patsy (early years) or, far worse, Millie Small. It has to be brought back into the originalcontext: they just took after the New Orleans r&b tradition and it was deemed as trendy and cool then. In any JA musical Stages, there are many songs that don't stand the test of time so well, and many others who do.
Let's not forget either that a lot of ska hits were actually instrumentals, not too surprising as they were mostly played by older, more proficient artists: musicians!