Hannibal Lector had little screentime, but he still proved to be an amazing and complex character. And how can a character be complicated and one-dimensional at the same time? The whole idea behind having depth as a character is that there's always a sense of unpredictability as far as what they'll do next, which is how character development occurs. That's why Spike from Cowboy Bebop was so universally praised or Shinji from Evangelion always raised the question of whether he'd chicken out or step up to the plate when fighting the Angels. One-dimensional characters have no range in their personality, so you always know exactly what they're thinking and what they're gonna do next, even if they don't tell you. Johan was an absolute wildcard and, to tell you the truth, he probably would've been better without the whole Nazi experimentation backstory. As a freak of nature and a drive for both Tenma and Nina's actions throughout the series, he was perfect. The backstory was essentially an excuse to humanize him when it really just added to Nina (since she was the one who actually got experimented on and Johan was pretty much screwed up from the beginning). Does that give him much personality? Not really, but L from Death Note didn't have much personality and most people love his character more than Light Yagami. It's cause as an antagonist (a character meant to be an obstacle for the protagonist to overcome and give them a drive in the events leading up to their final encounter), he fulfilled his purpose to a T and then some. That's why Johan is so well-loved as a villain and given so much praise as a character. The development wasn't always there, but the depth was constantly present.