yes, it will just end up in puddles under the mat which is far more hassle, and bad for the horses respiratory system. alot of mats are very heavy so cleaning out from underneath them would be a nightmare. I had mats in my stable and every month or so I cleaned from under them and disinfected the floor...and I dreaded doing it because It was so much work!
Mines a fairly level base, does drain but v. slowly.
I've got mats down and get away with swilling once every 6 weeks or so and a proper clean once a year
I just have the brick missing in the back wall and one in the side wall (its the end stable), my door has a row of bricks stopping it draining that way.
I leave the door open too so on the odd occasions the sun does come out that helps it dry up too