Disregarding the fact whether Mewtwo is low tier or not, that tier list really doesn't reflect our metagame. Like others have said, the Japanese metagame is really different from ours. Their stagelist, playstyle, and characters they use are all different than what we normally see. According to the Smash wiki, a tier list is based on several different things: the metagame, effectiveness of character's strategies, moveset and statistics, matchup spread, and tournament results. For example, in America, we have players like ESAM who get great tournament results, which is why Pikachu is so high in our tier lists. As far as I know, Japan does not really have any notable Pikachus - which is why he's so low there. Japanese players tend to play defensive and use Omegas a lot - which is why Sonic and Duck Hunt are so high in their tier lists.
I feel like you all are jumping to conclusions a bit too quickly. It's nice to compare our tier lists with there's, but to say one is more accurate than the other is a bit ignorant. Like I said earlier, we have to take into account how different our environments are. The characters that we think are good and bad may be completely different than what Japan thinks simply because our metagames are so different.
Besides, both tier lists aren't 100% accurate. Things change all the time.