Poets can do this too, and have been doing this long before typecasting was patented. Remember Edgar Allan Poe? The guy who wrote dark poems like "The Raven" and "Annabel Lee"? He also once wrote A Valentine.
And then there's ee cummings, who usually wrote poems with a gentle tone, writing something like this.
The extremely high-brow T. S. Eliot was famous for writing about nihilistic despair (his early work, notably The Waste Land and The Hollow Men) and religious faith (The Four Quartets.) He also wrote the entire libretto of Cats as a serious of comic poems while relaxing between his more serious work.