I was a big fan of the Smiths when they were a going concern. If you had asked me why he was important back then, it was that he was represented empathy and a special concern for the shy/lonely/misunderstood etc and they had some great tunes....it was all good...
But when he went and revealed himself to be an intolerant shithead, it immediately de-vibed the whole shebang. Sucked the fun. Poured piss on the bonfire etc
He was too personally entwined with what made him 'good' I think. There are loads of musicians (writers/painters) etc who are loathsome personalities but their music is not bound up tightly with what they are like at home. With Morrissey, it's all mixed up.
So the more he reveals his grimness, the more it devalues his work
(Obviously the swan dive began in the early nineties as well...)