Really, I don't think there's a big difference in making OCs for fandoms and creating OCs for your own worlds. I think the reason that fandom OCs tend to get a bad reputation has to do with the fact that most people's first experiences with them are those that are written by inexperienced, usually very yong writers whose self-inserts or OCs lack discernible character flaws and have no limits and therefore lack definable traits. Developing your own characters is commendable. I tend to prefer focusing on canon characters within my fandoms, but I sometimes have created OCs for a purpose, and I commend people who can do it more comfortably. There's nothing wrong with self-indulgence, but I think that you gain more from it if you're giving yourself and opportunity to develop a skill while you are working with it, such as authentic character development that includes the kinds of things I said some OCs people make in their first attempts sometimes lack.