How safe is the use of mesh in tummy tuck?

I have not used mesh in diastasis repair, and I have not even used mesh in hernia repairs.  The abdominal wall can usually be manipulated to provide reinforcement.

unless you have a hernia which needs to be repaired with a mesh, there is little reason why you would need a piece of mesh for a TT

There is no use for mesh in a tummy tuck.  If you have mesh from a previous hernia repair that is fine.  If you need mesh to repair a hernia you should not be having a tummy tuck at the same time.