I can think of three champion who are better suited to ba a captain than Yasuo

I mean, in-story the point is that Odyssey Yasuo is a character akin to Peter Quill or Han Solo, i.e. a bit of a jackass who finds himself in a leadership role he's totally unsuited for.

Sivir got a skin this year.

Why is Sivir here? Are you forgetting:

1. She has more skins than Yasuo.
2. She just got her Pizza Delivery skin in April.

I don't think she qualifies as a champ who could use a skin.

None of those fit the general theme of the odyssey though.

Because they are all waaay to serious to fit into a crew with Malphite,Ziggs and Jinx(Sona is allowed to stick out a bit,since she is a new recruit and is basically forced to ally with the odyssey).

Yasuo fits because even though he can be calm and serious,he also has a laid back side to him,which makes him a far better pick for the captain.

Also Sivir would be out of question because the team has to function in the new gamemode as well and having 4 ranged champs would be too imbalanced.