That is a fascinating question.
The problem is, it leads to other questions. Are these the only two options? What does the evil consist of? What does the 90% consist of?
These questions make it unanswerable. I can say that I don't necessarily see simple existence as a good thing in and of itself.
Perhaps a better question in context would be, did God choose a world with 10% evil over no world at all? And if so why? And remember to ask the same qualifying questions.
I define good as constructive and evil as destructive. In most conversations of morals, the wording is usually benefit and harm. The 10/90 split didn't come from me, BTW. That was from the person I was replying to. That said, this is how I see it:
The goal is not just good, but very good.
Genesis 1:31:
And God saw every thing that he had made, and, behold, it was very good. And there was evening and there was morning, the sixth day.
Perfection = good. Perfection + Imperfection that improves over time = very good. The one is quantitativley more than the other.
God already has perfection, that exists in the divine realm. Hosts of angels on a procession, praising and doing God's bidding in perfect unison with no errors in anyway. That's good. But creating something flawed, and then teaching this flawed thing to repair itself? That's amazing! That's like the difference between making a picture and making a puzzle. Or maybe writing a story compared to writing a riddle. Or maybe a map compared to a scavenger hunt. It takes much more skill to create both the flaw and the remedy, rather than just perfection. Creating all three: perfection, the flaw, and the remedy? That's very good.