Hello, I am currently in Mianyang and want to visit JiuZhaigou

Yes, you don't need to go to Chengdu first. There are direct convenient long-distance buses between these two places.
To JZG Goukou Bus Station, there is only one direct coach scheduled at 09:00 from Pingzheng Bus Station. It costs CNY 98 per person.
To JZG County Bus Station, more bus schedules are available. You can take it before 12:00 at noon. The ticket fare is CNY 88 per person.
In addition, the official website doesn't have English version. If you have Chinese friends, you can ask help for them. If not, just buy the tickets when you arriving at the bus station. Remember to get to the bus station as early as you can. :)