I don't see how she is a 5/5/5 but my current rating on her could perhaps use some tweaking. In any case I have her marked at a 3/3/5 (so that means 11/15) which I think is pretty reasonable. If you've got an argument for bumping her up or some stats screenshots to share that could change my mind, go for it.
I wouldn't focus too much on the out of 15 part since it's a team composition flexibility strength rating. And really, at 11/15, that's actually very good already. I got Orianna for example marked as a 4/4/3 and Jax as a 4/3/4 so they got the same 11/15 rating that Sona has and yet Sona is a higher rated helper than either of them are rated for absorber, damager, or helper.
Ziggs is a 2/5/3, which amounts to a flex strength rating of 10/15. Which goes to show you that a flexibility rating is not all that important(because he actually has a higher win rate than Sona does). However, I have to mention, I think ease of playing the champion has more impact on their win rate than their power when played well. There are a lot of casual players playing ARAM and they don't bother to learn details about most/all the champs they play so it's understandable that easier champs would have higher win rates.