Taskmaster is severely hindered by his personality, he has very little ambition as a fighter and he doesn't want to work for a win. If things get hard he starts looking for an out, which means the majority of the time he fights half hearted, while other characters put everything they have into it. We've never really seen what he can do with the chips are down and he has no options but to fight and win. Is it with in Taskmaster's ability to beat Deadpool? Sure, but it is more effort then he is used to expending and that puts him at a disadvantage. Maybe if Tasky wasn't a shameless opportunist he'd have a shot... but it's like the old saying goes, "When the going gets tough, the tough get going"... I guess.