Mad Maggie looked like an amazing Legend when she was first introduced, but it was quickly realized that wasn't the case. The Legend has struggled since her inception and things like a buggy ultimate doesn't help.
Mad Maggie's passive and ultimate has some major inconsistencies. Players have stated a lack of overall damage, sometimes the drill stops penetrating through walls, or the ultimate won't stun enemies. One Respawn developer was tweeting about some fun streamer shenanigans when a random player asked if the team was aware of the Mad Maggie ultimate.
The good news is that Respawn Entertainment is aware of the problem, the bad news, it doesn't sound like a fix is on the way. And it seems Mad Maggie needs more than just an ultimate improvement. Her entire kit doesn't do well even with such a heavy shield meta. Her abilities were supposed to counter Gibby and now Newcastle, but she can't really push these Legends down the board.
Her pick rate has been low for some time, and there are Legends that do her job better. Until Respawn reworks her kit or makes it more powerful, Mad Maggie is going to stay toward the bottom.