Clearly no one meant IT just had to be mlp.... obv you need other interest too or that will turn boring just talking about one thing... JUST it helps if they like mlp because mlp is something you really enjoy and you want to find someone that enjoys it too.
Also, what about someone with a very similar personality but doesn't really like mlp. Would you turn that person down because he/she isn't a fan of a cartoon?
Point is. Whatever common interests two partners may have, whether it's MLP, music, art, politics, other cartoons, it helps build some extra ground for the foundation of the relationship