How worse you might ask? Well, for starters, she intends to absorb the powers of the Samiyan Sea Crown as well as the Trident to effectively become more powerful, desires to completely ravage the world and existence itself, not content with merely ruling over the seas or even the planet or universe, she commits a lot of mass-murder, including a rather ghastly version of Ursula's soul steal technique where she actually absorbs the souls of merpeople, octopids, moray, sharkanians, and the like, has zero concept or belief in morality or goodness (being a huge nihilist in other words, to Kefka or Joker or even Carnage levels). She also would be the reason behind the events of Atlantis' destruction as alluded to in the Marvel Comics, and she has a grudge against Ariel due to the latter, via time travel, sealing her away. Oh, and instead of simply trying to steal Eric's love, she would outright murder him, alongside all Ariel cared for (all her friends, namely), and to make matters worse, she then would possess his body and then force Ariel and Melody to fight his corpse. Oh, and her "human form" would have been due to absorbing Ariel's shadow as well as trace elements of Eric's subconscious memories of his being brainwashed by Ursula. And she'd also have zero loyalty to anyone, if she accidentally kills one of her henchmen, any henchmen, like what happened with Flotsam and Jetsam, she'd just joke about it and laugh off their deaths (Ariel needs an actual archnemesis anyways).
But hey, that's just my take on it, what would your take be on Vanessa's character if she were her own character instead of just a disguise for Ursula?