Yes, you can put wood mulch around them. "Miss Kim" lilacs normally bloom a little later than the traditional old fashioned lilacs everyone is used to. The best time to prune a lilac is right after it is finished blooming, and make sure it is before it small nubs form for next years flowers. If you prune too late, and cut these small nubs off, you won't have any flowers next year. Also lilacs like soil with a PH that is close to neutral or a little on the alkaline side, between 6.6 and 7.8. If the soil is too acidic, they will not bloom for you.
Enjoy them - they have an amazing fragrance!
Val