Actually if only Oda hadn't put soo much shenanigans on her character, she has potential to be good character.
Imo he should be the motherly type. Make her like a wise grandmother, remove the over plumpy fat physique and gluttony and hunger pangs crap. Make her a homy granny. Keep the sweet tooth, make her fight for something important. For example, the way Tywin Lannister fight for his family. A touch of dry humor like Oleanna Tyrell would be nice imo.
If you gonna make a magician concept, make her a magician. No need to make her an iron balloon who excel in physical stats also. Optimize her role as elemental wizard. Stop making Prometheus, Zeus a personality of their own. Make her literally her personification. Every time prometheus talks, it's Big Mom talking. No need to make the fear-based soul-sucking ability, but remove the restriction of her elemental control. Make her in absolute authority and over any element she managed to give soul to. Giving a soul to fire = greatly amplified resistance and even healing against fire-based, heck even heat-based attack.
Next level, make soul-link ability. Like if the enemy is a logia, BM giving soul to a piece of that logia body would gain partial control of the body part that is given BM soul.
Somewhat weaker physical durability due to old age and weaker str? Not to worry. Akainu would be still frustrated if BM give her soul to his magma, and proceed to getting herself healed and more healthy everytime magma hits her. And proceed to control Akainu's right arm to break his left hand or attacking his friends.
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Btw on topic, worst character ever? I dont think so. Imo many worse, maybe gary stu characters like Kirito from SAO. Fairy tail, characters having no purpose despite their main role in the fiction, etc