Mat is used in polyester laminations to improve the bond between
layers of woven glass. Polyester has poor bonding qualities and that
is why mat is used.Woven on woven in a polyester matrix has poor peel
strength.
Epoxy has greatly superior bonding properties and mat would be a waste
if used for that reason. Mat can weaken an epoxy laminate properties
by increasing the distance between the layers of woven.
(woven includes directional).
The only reason to use mat is to add bulk - thickness - stiffness or
resistance to impact.
For that purpose, mat shoud be used towards the middle of the laminate
stack.
It does not add strength.
This happens in small boats in which the skins are too thin for
adequate stiffnes or resistance to puncture.

Jacques from bateau.com