Something that's faddish is in style, often for a brief length of time.
Flapper dresses and bobbed hair were faddish in the 1920's, and long hair and bell bottoms were faddish in the 1960's and 70's. Among elementary school kids, it was faddish in the early 2000's to collect and trade Pokemon cards. The adjective faddish comes from the word fad, which was coined in the 1880's, either from fiddle-faddle or from the Latin word for "stupid," fatuus.