When people lose to Doomsday, it is because they get flustered by his rush down, and stop blocking ES and teching throws. They start jumping up back or back dashing, both of which get blown up by MB venom. Against DD, you have to be patient and have great defense (easier said than done against his rushdown), and if you have that, you can do well. DD is a great character though, due to his rush down, and his venom takes you to the corner where on every knockdown he gets to reversal your wake up. Brutal.