For what it's worth, I take back one thing -- I had another look as to whether he was widely considered serious or not. In fact, Hamish Henderson likes to suggest the folk-poet may truly have been a satirist all along... perhaps just playing it "in character." He cites a popular tale that McGonagall was once seen to be leaving a performance in Edinburgh with a "satiric smile". (Alias McAlias p305 & elsewhere). But it does seem he was buried in a pauper's grave. Some time after his death, I believe.