Couple issues here. If the old mastic is hard and chips off, you may end up with a crunchy sounding floor.
The subfloor for laminate needs to be flat within about 3/16″. If you end up with hollow spots, this will weaken the laminate joints and they could fail. But if the floor is pretty flat, the foam underlayment will absorb minor imperfections.
So I would sand off the mastic until it is reasonably smooth. Shouldn’t be too difficult with a palm sander and it should go pretty fast.