We used to have a guest room/office that included a twin mattress, which we styled as a "daybed" with bolster pillows. We always had trouble making it look like a sofa until we stumbled across a secret trick. We added striped bedding and, suddenly, it stopped looking like a mattress!
The striped pattern of the bedding subtly defines the edge of the bed and makes it look more like a cushion, less like a mattress. We've seen this trick used in other homes as well, using loose blankets, tailored slipcovers, or pin-striped cushions.