What if I were to tell you, that as a statistician, it can be statistically proven that the strength of a character is not a variable independent of it's status as a vanilla/DLC character?
I'll walk through it.
There are 24 vanilla characters in Injustice.
There are 6 DLC characters
there are 5 "top 5 characters" that categorize the strongest in the game.
Because the ratio of vanilla to DLC characters, we can expect the top 5 will be 80% Vanilla characters, and 20% DLC characters.
Expected: 80/20
Observed: 60/40
*BRB will do the Chi Squared later**
While that is a fair equation, you also have to consider before the 1.06 nerfs and buffs what the statistics were...
Who was top 5 back then? Memory serving...
Superman
Black Adam
Aquaman
Green Lantern
Wonder Woman
1.06 happened after all of the characters were released, if I recall correctly... So in fact, there was no incentive to buy DLC at that point and time. Lobo was low tier, Zatanna was low tier, MMH and Batgirl and Zod were considered "high tier" but no one began to perform with them, and we all know what happened to scorpion. So in reality, once all of the characters were out...
The top 5 was still 100/0 vanilla.
After 1.06, it changed. Up until that final patch, your theory about DLC being marketed as powerful falls short a month or two.
And even still, the top 5 isn't finalized. Hell, the game isn't even figured out yet. I do nothing but find tech, and even now, a year and a half later, I'm still finding new things with Bane. Some of them change MUs, some of them change the entire flow of the game. But still, I shouldn't be able to find some 130 techs with Bane, each unique in purpose and potential, and then still continue to find more and more. A year in a half in, I shouldn't be finding out Bane has P1 advantage vs Superman.... but we do.
So yes, for now, there is a DLC-dominant top 5. However, all of that is just one neutral shift away from changing. After all, we have two vanilla monsters (flash and bane) knocking on the front door. A single change, a single MU shift, and we could be seeing a DLC character drift out of the top 5.
Sorry for the rant, but when people complain about DLC saying its always made to be OP so people will buy it, it does bother me a bit because that's not always the case. Some characters do better than others, that's a fact that can't be denied. Some characters came in late, got explored further, and now its too late to change them. That's something we can't help, but its not something we have to just accept either. The game will die in a few months due to MKX-itus, but until then you won't see a day go by where the game isn't changing in some way, some how.