A class being unpopular doesn't mean it isn't balanced, it means it's not popular. There could be other reasons, such as a class being inaccessible for new players. Especially if it's then compared to other, more iconic classes. If you want to talk about balance you look at what the class can do in terms of output and relevant utility. If you want to talk about popularity and what needs to be done to attract players to a class, that's a separate discussion that may include class balance but is not limited to it.
