Pidgeotto could have defeated Staryu doesn't really have much to do with Staryu being unevolved. It had more to do with how it could take down Starmie, who hadn't battled at that point, relatively easily, so Staryu, assuming that she could have called it back again, would have been at more of a disadvantage due to how it had already battled against Butterfree. Pidgeotto barely took much damage, if any, while Staryu did take some damage and could have been tired out even after being in its Pokeball for awhile. While it wouldn't be a guarantee victory for Ash, I definitely don't think it was a fifty-fifty chance. It was leading more towards Ash given how the match was going.