So, if this is true (and that's a huge IF), then honestly...
I'm pretty okay with this line-up. Would I have preferred at least two of them be from unrepped shows (particularly at least one from Chalkzone)? Absolutely.
But just looking at what we got (again, assuming this is actually what we got), it's honestly not bad.
While not one of my favorite SpongeBob characters, Krabs is a main character who is apparently greatly requested, so that's fine. Zuko also makes a ton of sense, and honestly, I don't what anyone says, Iroh is an awesome choice; he's easily one of my all-time favorite characters from Avatar, possibly in my Top Five at least, and considering how many characters I love in that show, that's saying something.
The only one I'm kind of "eh" on is Rocksteady, and that's moreso because I'm just not that big on TMNT, so unless the moveset's really interesting or creative like April's was, I'm not going to be all that interested in a new Ninja Turtles character.
So yeah, two main characters that make sense, one awesome pick, and one "eh". Personally, I wouldn't call this "mediocre".
Again, assuming this is actually the DLC line-up.