Yes, but Carl isn't mid tier. He's A-tier, so he's not a part of the discussion. And you have a dp now, you're fine.
And I'm not sure about all around game for Tao, but from what I can see, her only viable damage option comes from taunt loop.
Yes, you can get it off of almost anything, but the execution has to be there. And her advantage may from CT may carry over, but her matchups with other characters are way harder that Lambda's.
I dunno. I don't main Lambda all, but I feel like things are way easier when you're playing a better character. And as for beating scrubs with mid tier characters you don't main, doing it with Lambda is probably easier that doing it with anyone else. IMO, that makes her high mid tier, even if it's just a slight advantage over other characters.
I will agree that there isn't much difference in characters who aren't top 4, but the difference is there, however small it might be.