The reason why Etemon feels like filler.

[Fantastische Hure]

First of all it's like the shortest arc in Digimon Adventure, but that aside...

Plz look at the other main villains. Devimon, Myotismon and Piedmon. A devil, a vampire, a creepy clown and a...monkey?

ye...you see?

Also he didn't really have a theme that sounded creepy like the other ones IIRC. He was just goofy. Felt completely out of place.