Oh yes, the harness can cause mats. Tuffy gets matted everytime he wears his harness, especially if we go on a really long walk. You aren't a bad mommy at all.. Sometimes even brushing regularly won't keep mats from happening. I swear that Tuffy mats in his sleep!
Another product I really like is the Leave In conditioner from Petsilk.. it helps cut down on the amount of mats Tuffy gets and it smells awesome!