Using mousse-hair styling product after hair transplant?

There is no benefit to using mousse after a hair transplant surgery.  If you want to use it for styling purposes it is up to you.

As long as you wait at least a week following your procedure you can use any styling products you wish.You want to avoid those products initially so as to not injure or dislodge any grafts.

Any sort of styling hair products should be fine to use once all your scabs/crusts have fallen off, generally 1-2 weeks after your transplant. There aren't any benefits in terms of helping your hair to grow or anything to that effect, it's purely for aesthetics.