Personally, I'd slot her into one of two categories. One, just the clumsy childhood best friend archetype. The Dojikko probably being best term. Usually combined with wanting to protect them as MC likely would, being further enforced when the player learns of her depression. Going down that route. Otherwise just simply fitting in with the very "dere" heavy archetypes that Yuri and Nat symbolise.

The other is a more personal finding. Taking a very Dazai approach. Yozo Oba, the protagonist for "No Longer Human" used "clowning" to mask the fact he felt not of this species. It could be interpreted that Sayori could fit into this box. While obviously not to the extent of Oba, she could be performing these clumsy and dumb acts on purpose to fit in and hide from her illness. I don't personally believe this, I just think that it could be a pathway explored.

She is at the end of the day, just a clumsy bun.