I would plan of going to jiuzhaigou in early Apr 13, is it an idea month? what is the weather?

It can be ok to pay a visit then, but it is not the ideal month there. It should be around Oct. and the early Nov.
Anyway, you can also enjoy yourself by appreciating the distinctive scenery in Apr.
A little cool weather, with the average temperature of about 10C or a bit higher, needs you take an overcoat and a sweater.