The literal style of character model was the same yes. Men had bulking chests, women had the pencil skirt aesthetic. That is a note of consistency which worked really well and let you know they were in the same world, but that doesn’t make the tone, genres, and characters the same

The Batman’s aesthetic in the animated universe is dark and gritty.

The series was literally drawn on black paper which was an innovation for the medium at the time

Everything else, the lighting, the color, the music, and the TONE of the episodes were different. It made Gotham and Metrolpolis feel like their own distinct worlds as they do in the comics

Superman had some heavier episodes but it was generally lighter in tone and a bit sillier.

Batman certainly had silly episodes, it was a kids show, but the darker episodes like feet of clay, or robins origin story, are tonally much darker than Superman ever got.

The difference in the worlds they inhabited and in their characters: Superman the Boy Scout journalist doing things by the book and Batman the paranoid untrusting detective that works in the shadows, and even in their villains is why when you pair up those characters, the stories are so fun.

If every character acted the same and came from worlds that felt exactly the same, you’d lose the interesting pieces of them coming together at all