Pokémon is the trademark (see e.g. https://en.wikipedia.org/wiki/Pok%C3%A9mon). Pokemon is common spelling in countries where keyboards don't have é and/or where people aren't used to inputting é.
It is common for brands to localize their names. E.g. Axe (the deodorant) in some countries is branded as Lynx.
It is common for brands to localize their names. E.g. Axe (the deodorant) in some countries is branded as Lynx.