It's not something I should hide, in that it's nothing really to be ashamed of, it's just something I happen to hide anyway. I'm a pretty private person, so I keep most of my activities and interests to myself - even my parents don't know half the stuff I'm into (lucky for them it's nothing illicit ;p). And to be honest, if someone meeting me for the first time asked me about my hobbies or video games that I liked, I probably wouldn't mention Pokémon.
I personally don't feel that liking Pokémon or other entertainment aimed at children says anything negative about someone's personality, but others seem to think that it's a sign of immaturity or some such. Which is untrue, of course - lots of smart, mature people engage in "childish" activities and there are certainly much worse habits to have - but rather than dispute that with them I just keep playing it without announcing it to everyone I know. Basically, even though I know there's nothing wrong with playing Pokémon, I wouldn't mention it at a job interview.