Utah has 4 players who can all shoot and are excelent offensive players, so grabbing a defensively versatile and limited (but low usage also) Mbah a Moute makes sense. Especially when he's unlikely to fall a full 30 slots till Utah's next pick.
He's also been far more effective at the 4 than the 3 for Milwaukee.
That's because the Bucks are an already offensively challenged squad, they do much better if Mbah a Moute is at the 4 so they can play a shooter/scorer at the 3 position. Better to be undersized than have too many non-scorers out there, especially since Mbah a Moute is a very flexible defender.
I don't think Utah will have a problem offensively with Mbah a Moute at the 3. Meanwhile his rebounding is stronger relatively there and his defense is still very good.