Agreed, it's beta, and the MM isn't optimized for skill level or party size, I'd imagine. I'm seriously a pleb, and I've played about half my games where I am with amazing players...and the other half with people more my skill level...it happens in beta. 🙂 Just be glad you're playing!
Also, there are already crazy long queues for 5 stacks. If they would only match them with other 5 stacks it would be an eternal queue.