"If" she can kick anyone around 70% near edge of stages with poor DI, then yes her sweet spotted predictable floaty forward and back airs are fine. Her extremely poor knock back and punishable down smash can't be used out of shield that easily. Up Smash... maybe.
Mewtwo has guaranteed finishers on all characters except fast fallers with a completely average ranged grab that seems slightly disjointed. Mewtwo actually has a couple of reasonable combo starters with Down Tilt and throws depending on the character's fall speed. If Mewtwo can get inside most characters, he naturally has a shield pressure game between low attacking down tilts and grab mix ups, on top of a DJC forward air to grab mix up, AND a slightly longer range confusion through shield that catches people off guard.
Mewtwo is big, but it doesn't matter most of the time, you just gotta space better. His size makes his neutral air really good for punishing poorly spaced aerials, people on platforms above you, and controlling where people decide to move away from you.
I wish I could make Mewtwo mid tier, but he's definitely low tier. I proved that Mewtwo is not "awful" I've beaten several big name players in tournament matches with Mewtwo. (Tournament: Chu Dat, Plank, King, Forward, Wobbles, to name a few. Friendlies: NEO, HugS, DaShizWiz, Wife to name a few.)
Pichu simply doesn't have the same options as Mewtwo. The only reason people really hold Pichu in a higher regard than Mewtwo is that Mewtwo can't be played with orthodox mind games. Pichu has a Shffl game, a strong Up Smash and chain grabs on spacies on FD. Mewtwo can't play like the majority of the cast. Mewtwo needs to simply be played very carefully and punish efficiently.
Pichu is the worst character in the game simply because his projectile is worth blocking, very low pressure and he hurts himself. Pichu hurts himself when using half of his good moves and recovers. Pichu is the absolute lightest character in the game with VERY LOW priority (Worse than Mewtwo), worst grab range, and uses nothing but FULL BODY ATTACKS. Aerials can only trade, especially with Pichu's low priority which means Pichu almost never comes out ahead. Pichu isn't even that fast, he only has quicker l-cancels and landing lag, but his run speed is actually pretty lack luster. Pichu also gets chain thrown and grab comboed by many high tier characters. Overall matchups against most characters are actually very bad. Pretty much Falcon and Falco are Pichu's best matchups against the High Tiers, and Falco is only a good match up on FD.
Mewtwo- Limited combos, but they are still very effective though. Mewtwo has an actual projectile that can pressure the opponent charged or uncharged. Mewtwo's shadow ball can pressure opponents even while it is being charged for shield pressure mix ups. Mewtwo can escape from most characters and is almost impossible to corner unless you are Falco, Fox, or Falcon, and to a lesser extent Sheik and Marth. Mewtwo's Teleport is invincible and coupled with a super double jump, has one of the best recoveries in the game. Mewtwo without a second jump is in trouble in some situations though. Mewtwo cannot be chain thrown by any character except for IC's and Marth at low %. Mewtwo's matchups are much better rounded against the higher tiered characters with the more difficult matchups being Fox, Sheik (Depending on play style), Falcon, and the Marios
TLDR: Zelda actually blows if you know she can't do anything else in the air.
Pichu= No priority, Lightest Weight, hurts himself, poor grab, difficult to land finishers.
Mewtwo = slightly better priority, little heavier, Big target but still mobile, Well rounded shield pressure game, UNORTHODOX.
If you want me to go into more detail, I can most definitely do so. You can ask me more specific questions/match ups by PMing me.
I can also address why Mewtwo is better than Bowser, Kirby, Ness, and MAYBE Yoshi, but I haven't really looked into Yoshi as much as the other characters in the game. I can see Yoshi being very tricky if played insanely well though.