The lackluster roster was directed at Smash 4, not Brawl. Brawl's roster what good because despite the fact that my favorite character wasn't in and couldn't be replaced, Had Mewtwo never been in smash Brawl's roster was a huge step up.
In smash 4 however, Sakurai knew the significance of Mewtwo and others, we expected more. But then what do we get? CRAP!
the initial roster of Smash 4 is lackluster but now that Mewtwo is in the roster is good. and yes, i basically play smash for one character:Mewtwo, that's my preference. and to be honest i view it like this:
So there is this dish you are cooking, (Characters) so at first you get one topping, some seasoning, the sauce, the added flavor, the accessories, however, there is one thing missing from this... The base item on which this is to go onto (Mewtwo). so instead you just decide to clump all of the toppings together with super glue (clones), and call it a day.
However, neither the toppings, nor the base item are any good separated, so when you try to accommodate for this by adding in super glue it becomes a mess. The point being that all of the Smash content and characters are of little value but they together lead up to what will make the entire game good alone... Mewtwo.
That's why without the Mewtwo DLC, smash 4's roster sucks to no end.
And Mewtwo is the only character who can solidify the game which is why we don't need excess (other DLC)