I wouldn't say that games in this format are at all decided on who goes first. I've won plenty games going second, and lost games going first as well. Mewtwo certainly doesn't make the format more luck based either, in fact I think it adds an element of skill and makes games more based on outplaying your opponent. You have to actually think about when to play your Mewtwos, as well as your energy attachments. It's definitely not about who gets the Mewtwo first, or who attacks second, it's about who's smarter about how they use their Mewtwo. I mean sure, there's always luck involved, but Mewtwo coming into the format definitely didn't make luck a bigger factor.