Actually, that would be "successful". The success of an anime is dependent on the tastes of the population, not the quality(note the lowercase). Usually the population likes quality though, so it works out.
However, sometimes the population, for various reasons, prefers something with less quality than something with more.
It's like comparing a popular rap song to Rachmaninoff's 3rd Concerto. The rap song sells more (these days, ignore past sales), but that doesn't mean it's any good.