Bill Mantlo was a workman (I say that knowing full well you dislike the term). He knew how to put the parts together and sometimes wrote terrible comics which are far outnumbered by his of average or better quality. His reputation was destroyed by terrible blogs by vain comic pros who chose to target him and his contributions to comics because they are insecure and need to insist something else is bad to build themselves up. Mantlo was the perfect patsy - he's someone who can never ever reply or dispute warped versions of events by Jim Shooter or Tom Brevoort that are created to make Mantlo look as unflattering as possible so they can insist that they themselves are better than THAT. Whenever Jim Shooter stepped outside of Mantlo slander to share similar stories about other creators - like when he accused David Anthony Kraft of "ripping off Blue Oyster Cult lyrics" and how the band were so angry about it and were going to sue Marvel were it not for Jim Shooter's charisma and peace-making when he managed to convince them not to pursue litigations... which DAK replied to showing a photograph of him with the band and mentioned how he had always had their blessing from the get-go, how he even credited Eric Bloom in the issues, and how the story was actually original and only used ideas and names from songs. That was the same standard of accusation Shooter's claims of Mantlo stealing other people's scripts from the office and using them as his own or Mantlo ripping off Harlan Ellison so bad that Harln Ellison was going to sue. Mantlo can't reply, so they all stand uncontested. If Shooter went back and started telling his version of the Blue Oyster Cult story now - it would likely be treat the same way and damage Kraft's legacy because he is no longer able to retort and show that Jim Shooter is a fantasist. And it's not just Jim Shooter. Tom Brevoort, Mark Waid, Kurt Busiek, Erik Larsen, Marc Guggenheim - they all tell very similar unflattering stories about Bill Mantlo (which is fascinating considering Busiek was the only one likely to have ever met him). Some of them are even fucking published as introductions to Marvel Masterworks collections of Mantlo material. There's no stories about Mantlo coming from Roger Stern who has a very reliable memory for these things, there's none from John Byrne who is fucking terrible for venting about people, there's none from any number of the artists he worked with. It's always solely the same type of person who does so, the person I abhor, the "Comics Were Crap Until I Fixed It" sort. Mantlo is an easy victim for abuse and that is all he is to them.

Like in South Park when they had a show at the planetarium that was Laser Kenny Loggins and Laser James Taylor?

Yep, he was always the guy doing fill-ins. Almost every time Claremont needed a fill-in during his early X-Men issues, it was Bill Mantlo doing it. A lot of the books he wound up writing were also ones that nobody wanted to write (Champions, Marvel Team-Up, early Spectacular Spider-Man, mid-70s Iron Man, the licensed stuff like Rom, Micronauts, and the less flattering Human Fly). Would have loved to see where he went in the 90s, I wonder what of those characters/titles he'd have ended up writing. Cable, War Machine, Night Thrasher... surely, he'd have done a Nomad fill-in issue at some point.