I don't think the players on your team are really any worse when you duo queue (assuming you're not super high ranked), you just have someone else to rage about them with and point out their mistakes so how bad they are is more noticeable.

When you're solo queue, say you're bot lane and your random pub completely fails a jungle gank top...you are often none the wiser. However if your duo q partner is top and the jungler messes up badly they're now an awful player in both your minds. I also tend to not try as hard in terms of leading the team and what not when I duo because I feel like we can pick up the slack but then often a pub will throw the game because I didn't type out as much strat to them.

It doesn't really make any sense to say you get worse players because the difference between a the players you get as a duo and solo are a matter of a couple wins here and there (+2 wins = 25 rating) which in LoL doesn't amount to meaning anything skill wise.

From my experience anyway when I duo on smurfs and fuck around we always say wow our team was really bad etc...then when I duo on my main account and try properly we win nearly every game until we start facing the players around our skill level.