When you level her up it becomes almost negligible (goes from Def Down [Medium] to Def Down [Small]), also it functions like any other status-down where you can remove it or "block" it with skills like normal Amon's. In contrast, Beelzebub's buffs can make pretty much any garbage team good enough to make it, I picked her from the pre-reg gacha and she carried my halfass "no idea what I'm doing" newbie team like forever.
I'd say Beelzebub is legit one of the best characters in the game, pound for pound. In the beginning you don't really care about the defense down and by the time it matters you understand the game well enough to plan around it. She's just a solid all-around pick.