In terms of overall performance, Andrelton Simmons has arguably been the Braves third best position player this season behind Justin Upton and Evan Gattis. As expected with arguably the best infield glove and arm in the league, Simmons' value is almost entirely tied to his defense. Currently, of the eight lineup regulars only B.J. Upton trails Simmons in wRC+ and I do not expect that to last much longer as Upton seems to be getting things together.
In terms of what you look for from a leadoff man, I do not see one quality that Simmons actually possesses. Those qualities are an ability to work the count, get on base, and run the bases well. Of course, there is more to it than those three factors, but specifically in leadoff men those are three attributes you would like. It is arguable that Simmons is the worst on the team at the first two and is still pretty poor at the final factor. While he is not as bad of a base runner as some players on the squad, he is a far cry from "good."
Simmons does not strike out or walk more than 10% of the time. Having a low strikeout rate is a good thing but not if it is in coordinance with an absurdly low walk rate. He has the hand-eye coordination to make contact almost at will, and while there has been a plethora of people complaining about strikeouts this season, making contact is not always a good thing.
The main reason I do not mind a lot of strikeouts is that often times, and this is the case for the Braves, high strikeout rates come with high walk rates. Conversely, low strikeout rates often come with low walk rates, and low walk rates are not a good quality for any player to possess. Some are able to hit well enough to counteract their lack of free passes, but soft-hitting middle infielders are not those types of hitters. When looking at why strikeouts are not such a big deal, look at it from a macro level and understand that Simmons' struggles are what the people who are okay with the K are pointing at. It is not that people do not care about strikeouts, it is that they like walks. And the fact that they like walks so much makes them not worry about the strikeouts, because the fact that they see more pitches is a good thing in the long run as long as they are willing to take free passes.
The order in general is not too big of an issue, but it does provide some value to get it somewhat right. While some ask "who should lead off instead of Simmons?" the answer is simply "anybody but him."