That’s actually a funny way to interpret the Harry Potter films, but the joke works better as a criticism of Voldemort’s leadership than as a literal measure of his power. Bellatrix directly kills Sirius Black in Order of the Phoenix and Dobby in Deathly Hallows: Part 1. Voldemort’s on-screen record is harder to count because some of his most significant murders happened before the main story or off-screen. He killed Harry’s parents, murdered Cedric Diggory through his command to Wormtail, and killed Snape through Nagini in Deathly Hallows: Part 2. He also ordered or enabled numerous other deaths. The more interesting comparison is that Bellatrix is often shown doing the dirty work while Voldemort cultivates the image of an unstoppable Dark Lord. She is personally responsible for some of the story’s most emotionally devastating losses, and her death at Molly Weasley’s hands shows that even one of Voldemort’s most devoted followers can be defeated by someone she underestimates. Voldemort’s own downfall is even more humiliating: after decades of terror, he loses because he misunderstands the Elder Wand’s allegiance and underestimates Harry’s choices and the protection created by love and sacrifice. So, is Voldemort a fraud? Not in terms of magical power he is genuinely one of the most dangerous wizards in the series. But his arrogance, obsession with immortality, and inability to understand love repeatedly undermine him. Bellatrix may have the more memorable on-screen kill count, but Voldemort’s real failure is that he mistakes being feared for being invincible.

She didn't kill his parents, and wasn't even trying to. She was (however misguidedly) in love with Voldemort. He had disappeared and she believed he had been captured by a regime which would torture him and then use Dementors to destroy his soul. As Aurors at that time, the Longbottoms were either torturers or the colleagues of torturers, something like Stasi officers, so Bellatrix had no compunction about torturing them in an effort to get information out of them about where her love was being held prisoner. Of course they didn't know but she didn't believe them and kept it up until they got brain damage. Her behaviour was ruthless, but it was logical, and motivated by love. Later on she was a deranged killer but that was after she'd spent 14 years being psychologically tortured by Dementors, so we don't know whether she was that crazy before.