I don't understand how this decreases Mewtwo's chances in fact I think it increases it even more. I'm going to copy and paste my reasoning from another forum as to why I think this could actually be good for Mewtwo.
"Considering Mario was bumped to 5, I can easily see Pokemon getting the same treatment. Of course, this isn't definite but it seems pretty likely. Mewtwo still has his own merits to get into Smash. He is still insanely popular and obviously still relevant with the newest movies and Pokemon X and Y. Not to mention, there is newfound potential with Mewtwo given Mega Mewtwo if his old moveset is considered lackluster by Sakurai.
Actually, I think Lucario being released early IS good news for Mewtwo. Sakurai probably knows already how much Mewtwo vs. Lucario would excite people. This case is actually starting to remind me of a certain happening before Rosalina was revealed. I don't know about you, but I really felt Peach was revealed for no apparent reason other than getting the veteran Mario characters set before revealing Rosalina (Super Mario anniversary was never mentioned). Just a theory but I think Sakurai is setting us up for something at E3 maybe. I wouldn't be surprised if we actually got Pokemon Trainer and Jigglypuff soon now. Anyways, if we do get a 5th Pokemon rep, it's probably going to be Mewtwo."