I don’t see the motivation for Olenna to be a mole when Cersei murdering her entire family is still fresh on her mind. I don’t see it. Also, it appears as if Dany’s ships were split up between the part that were going to lay siege to Kings Landing and the part that were going to Casterly Rock. The part going to Kings Landing were the ones attacked by Euron. The show indicated that they would be splitting up and that going after Casterly Rock would be somewhat of a surprise.
If there is in fact a mole, then the mole preferred that Casterly Rock be sacked as opposed to the Dornish and Tyrells laying siege to Kings Landing. Coincidentally, the part of the fleet not attacked is the part that’s going to be on the other side of Westeros from Dragonstone. Aside from the dragons, the remainder of Dany’s fighting force are now quite far away essentially preventing her from really projecting any force on that side of Westeros.
The mole is not a minor character and it’s not someone seemingly without ambition in Westeros (not Missanthei or Grey Worm) — that leaves Varys and Tyrion. Don’t forget the scene where Varys and Littlefinger come across each other in the throne room where Varys looks at the Iron Throne when Littlefinger asks of his motivation.