I can understand why you would say this, but I don’t really agree. There are a couple of complete filler episodes, but for the most part I think every episode is important. Avatar thrives on how good not only its main story is (I think the main story is good, but honestly is weak in comparison to the other things it does so well), but also on the strength of its worldbuilding and character development. Episodes like The Storm, Zuko Alone, and The Beach are amazing for how they teach us more about the characters and show us how they are changing. Avatar Day, The Headband, and The Avatar and the Firelord give us insight into the history of the world, explaining the background of how it came to be how it is, and how the world outside of our main heroes currently function.

It’s a show with just 61 22 minute episodes, telling one unified story. It’s very much not the anime that keeps stretching out plot lines to fill up its seasons, or that stalls for time while waiting for the manga to catch up.