It's not only about generals, it's about troops AND their numbers. On Merilor he WAS better than Demandred (all tough he was severely crimped by his fixation that his opponent MUST be LTT), but not extremely so. And Dem had better numbers (conventional army AND in chanalers) more rested troops and probably better logistics.
That beside all those advantages Mat was able to match him move by move and fight with one hand tied behind his back (he didn't commit Seanchan except for message dilivery for most of the battle) does mean he is a better commander.
If they had met with similar numbers in similar situations with no chanelling involved he would have beaten Demi with great certainty.