It's fairly easy but time consuming to get to Mianshan Mountain from Pingyao using public transportation even though it's only about 30 km. The trip takes about an hour and a half each way.
First stop is Jiexiu and then Mianshan.
1) From Pingyao the buses leave from the main bus station a few blocks from the North Gate. The first bus to Jiexiu is at 7:20 am and then there are buses every hour. Cost 10 CNY.
2) In Jiexiu, tell the driver "Mianshan" and he will drop you off at a little bus terminal. Buses to Mianshan leave frequently and cost 5 CNY. The last stop is the Mianshan complex.
3) The return bus to Jiexiu leaves from the same drop-off point. In Jiexiu, tell the driver "Pingyao" and he will drop you off at a little bus terminal but you have to cross the street and go down about half a block to the left to a different terminal. There are little holes in the street divider that you can crawl through in order to cross the busy street.
4) The bus from Jiuxiu will stop close to the departure point near the North gate but not exactly at the Pingyao Bus Station.