Pokemon is a fantasy setting like any other, it needs internal consistency to be enjoyable. Most of the time it has that, but some Pokemon fall outside that internal consistency (ie: they didn't bother), those Pokemon are not generally liked.>>39947818
Science fiction is a subset of fantasy. In fact, Pokemon is a sci-fi setting. In a sci-fi setting, you still have internal consistency and lore. You can have something that can't exist, such as time travel, and as long as you have rules for how it works within the setting, the audience will make that leap of logic and accept it. You can't however, change the rules of time travel willy-nilly within the setting.. it needs rules or the audience will have to make a new logic-leap every time the rules change. The more logic leaps, the less believable the setting becomes.