That's a pretty close-minded way to generalize antagonists as just being whimsically belligerent. As if being a villain is synonymous with acting like a rabid dog without any sense modus operandi or unconventional ethics. Especially concerning someone with as much depth and uniqueness as Big Mom.
On such, I recall the very first thing you are adviced on to create your first villain, is to draw inspiration to parallell the hero in some manner and give them a goal to accomplish, something they sought to do and made bad decisions. Like heroes going to far on destroying evil often become the same evil in a different element because they destroy according to their perception.
Keeping this to Oda, he did partake in an older anime prior to One Piece, Kenshin. One of the final villains of the story was Anji, a fallen monk, A faithful monk who wished the best in the folk around him and was about to migrate with his spiritual family, when he unfortunately loses them to greedy politicians who wanted them dead to get a share of wealth from the new goverment. He blamed Buddha and sought to cleanse evil while shielding innocents himself, not realizing that he had become the evil he preached against. He works because Anji parallelled Sagara who had drowned himself in the very same pit of negativity that was self-hate and begrudging the Imperials.
If you want villains who are evil for the sake of being evil, you need not go any further than the Nazis in Black Lagoon. They could have been any kind of boastering freelancer group otherwise, but they were added to be nazis for the stereotypical evilness. It went nowhere with no motivation on their end except they wanted the same loot, and they were nazis- here, hate them. Props to the Vinsmokes living the comparison rather than swinging the label.
Evil needs a motivation, because the perception that makes it evil can vary greatly on the story. A hero who turned on his own his morals to accomplish his goals makes for a great villain. Maybe the murder of someone close to the actual hero was a necessity to spark a prosperity they both sought.