I really don't know what to say to this. I'm not sure by what possible measure, subjective or objective, TMNT could be considered to be superior to B:TAS in this regard, or any regard.
Don't get me wrong: I've a soft spot for TMNT, and an especial soft spot for April O'Neill and her gravity defying cleavage, but fun and all as TMNT was, it lacked practically everythng that made B:TAS great.
And by great, I don't mean just a good cartoon, but and excellent piece of televisual drama.
When B:TAS started, Batman, as a character, wasn't taken seriously at all. He was the Caped Crusader, but he wasn't the Dark Knight. B:TAS is what gave him that extra dimension. Sure, Conroy and Hammell gave us fantastic work as voice actors, and Hammell helped define The Joker as a character, but none of that would've worked if it wasn't for some of the best writing ever seen on television *ever*. Dini and his writers took characters who ought to have been at first glance the subject of ridicule, and gave them true depth and pathos. Few series can be said to have done that. Take, for example, "Harley Quinn's Day Off". It's far from the only episode, but it's one of the best by far, and the fact that she's an original character to the series doesn't dull the point. We have a character who is plainly a bad guy, but not an entirely unsympathetic bad guy, and we put her in an every-day situation where she oh-so-desperately wants to behave like any ordinary person would, but is forced to fight with persona, and, in a tragic fashion, ends up reverting to type in spite of herself. And right through all of this, Batman acts as the protector of the city, but foremostly as a stern but ultimately caring big brother to Harley Quinn.
The effect of a story like that is profound: whereas TMNT was black and white, B:TAS showed that regardless of the obvious shade on the spectrum of morality, others could care for the fate of others, and hope beyond hope that things could turn out better, regardless of whether they were aligned or opposed to one another. And Harley Quinn isn't even the best example of that, she just has the best episode.
TMNT, fun as it was, was so much more simplistic and juvenile than that. It was fun, but if I wanted constant fun back then, there was Earthworm Jim.
No. The critical element of B:TAS is that it offered deep narrative contrast: the characters felt like the could be *real*. That was something I never got from TMNT, and that is why B:TAS is better than TMNT.
BTW, Batman Beyond was a good series, once it got past the executive meddling and the writers were able to add some depth to the whole affair. It's a pity it ended before it had a chance to approach the heights of its predecessor.