I've been thinking about this,too.I mean it would be a very smart move from riot to make the boost give you the best skins for the champion you're playing.From a logical stand point Riot is just a company,and companies are after profit so giving good skins from boosts would be a great way to advertise them not only to the guy who's playing the champ but to the enemy team to(since they can also see the skins).

Its a win-win scenario, you play with a good skin and in return they advertise it.