They like "fluffy" for the same reason they like "curvy". It conjures images of cute, loveable, or otherwise "good" things, just as "curvy" brings to mind big breasts, big hips, tiny waist, and round ass. I think "fluffy" got popular due to the comedian Gabriel Iglesias using it as an adjective in the title of his stand-up movie. Regardless, the term has been so overused (just like "curvy") that now it is synonymous with ranch roombas, so all I see when someone says it is "fat sack of shit".