maybe i am just naive but i really struggle hard to believe that they did this just out of petty, jealous, junior-high spite.
I don't think this is run of the mill, "how come he has a fancier car than I do?" jealousy.
I think this is more like the Karate Kid/Cobra Kai series were everywhere Johnny goes, he has to see Daniel's face on a billboard, mocking him and reminding him of his defeat.
Think about it like this - all that Monica has is being a voice actor. She doesn't have the looks she once had, you saw her personality from the depo, she isn't that bright (based on her inability to even lie consistently), and she didn't finish college, so she isn't going to fall back on her education. I mean, Ron is the best she can do; she's been engaged for five years because she keeps trying and failing to find anyone better.
This is all she has, and all she is ever going to be, and yet, even when she is the main character, there is Vic, advertised on the box as the main draw - just like Daniel smiling on the billboard. Imagine having to go those conventions surrounded by all those people she despises and having to pretend to like them and then looking over and seeing 5 times as many people in Vic's line, and Vic actually enjoying it. Or realizing every day that she isn't successful enough as an actress to pay her way without those weebs, because nobody wants to watch a Super Fifth Bulma movie. When your whole sense of self-worth depends on one thing, and you consistently have to have your nose rubbed in being second place, I think that can warp you over time. And the fact that Vic doesn't even notice or care, and just goes on his merry [horndogging] way, just makes it worse. Having THAT kind of jealousy echoing around in your head for a decade is what drives you to behavior like this.
I mean, listening to her going on only voicing 'best girls' in HER DEPO WHERE THEY WANT TO MAKE HER LOOK BAD, that almost made me feel bad for her. Almost.
Best girls? Really? They're PIXELS for crying out loud, and you are just the voice - you don't even pick what they say. And frankly, I can't even think of an anime where there is one 'best girl', unless it is the only girl in the anime. But that's a separate thread. If I go off any more on that, I'll be as bad as her.

But it does highlight that she doesn't really watch, or care, about the anime she voice-overs, or she would realize how strange that sounds. So I guess that's another point - she's forced to work in an industry she doesn't even care about, just one more injustice to think about once she comes home from work and Ron starts humping her leg...