Most Helpful Guy
1900-1920 - Classical, Ragtime, and Marches, Big Band
1920 - Jazz became hugely popular
1940s/50s - Rock & Roll
1960s - Beatles.
1970s - Metal/Rock
1980s - Metal, early Pop, Hip Hop, Rap
1990s - Era of the Boy/Girl bands, Pop, Hip Hop, R&B, Rap
Basically everything was considered bad following rock and roll... Boomers loved it, the older generations hated it, Boomers hated Metal, when Jazz turned into more of it's wilder areas (late Miles Davis/Funk/Bebop) it started losing people, and basically became those who live for the big band era, and those who enjoy the newer stuff. Tons of people hate rap and don't really consider it music, granted, most pop now-a-days barely extends past a single pentatonic scale or one octave. What's good and bad is subjective anyway - most everything from the sixties until now is still on the radio, so that's good for people trying to make money, regardless of whether it's like-able music or not.
Also, that's very basic break-down, and a lot of those decades aren't set in stone for styles, groups here and there would extend into other decades, but that seems to be the general gist of it, last I knew