Correct.
First, NRS needs to analyze what makes the top tier characters top tier. Second, they need to think about what they can do to help the low tier characters.
For example, characters who have strong 50/50 mix ups (i.e., Batgirl, Killer Frost, The Flash, and Wonder Woman) are very good in this game. As are characters who can rush down and zone (i.e., Aquaman, Batman, General Zod, Martian Manhunter, Superman, etc.).
What does a character like Lobo or Shazam need to be up there?
Good 50/50s alone do not necessarily a good character make. (See: Catwoman)
The problem with Injustice is, as has been mentioned, that the characters are
so radically different and play in totally different ways, unlike, say, Street Fighter where each character will generally play in one of four or so styles, or Tekken where everyone effectively has the same style.
Even if each character becomes viable in their own right, with styles varying like they do, it's almost certain that at least a handful of matchups will be borderline unwinnable for one side.
In that respect, Raven is actually one of the most well-'balanced' in the game. She doesn't really have any match-ups at all worse than 4-6, and her very obvious strengths (awesome air control, good combo damage and great zoning pressure in Demon Stance) contrast perfectly with her obvious flaws (terrible range on normals, no low/overhead game and unsafe specials outside of Demon Stance), which some characters can take advantage of but none can fully decimate her with.
The likes of Catwoman and Hawkgirl, by contrast, are atrociously balanced.