Look, I do not typically involve myself in fandom drama, and believe me, I've seen plenty of petty squabbles in my years here in the DCA fandom. The fact that I even feel compelled to say anything at all should be a testament to how absolutely fucked up this situation is.
I hope that anyone who is faced with one of these DMCA takedown notices contests it. Contest it, contest it, contest it.
Momo does not own the rights to these designs.
The designs were created for a fan project, for free, based on the specifications as laid out by the creator of this fan project. If Momo wanted compensation for the art, she could have and should have asked for it at any point during the process.
I can't speak to why she didn't, but it seems like she didn't because she was doing something fun with her friend in the spirit of fandom. Now that the friendship is no longer viable, she is melting down and trying to take the rest of the DCA fandom down with her.
DO NOT LET HER DO THIS. FIGHT AGAINST THIS.
Momo's involvement in this project would be no different than if she had been an unpaid intern at a studio, producing concept art for a project based on the specifications and descriptions of other creators involved in the project. Just because she chooses to leave the project, does not mean she owns the rights to the designs she made while working on the project.
I get that it's a little murkier because this is fandom and she wasn't an employee with a contract, but the fundamentals remain the same. She created art based on the descriptions she was given by the creator of A?MLTYT and she even admits that she was asked to make changes to the designs to make them more accurate to how they were supposed to be portrayed.
If they were truly her designs, why would she be asked to make changes? Why would she make those changes at the direction of another if she was truly the one who created them?
I can almost guarantee these takedown requests are not being seen by human eyes at this moment. There is likely a system in place (probably AI) that is looking at the images and saying "Yep. Plagiarism." The best way to try and get actual human eyes on this matter is to contest the takedowns, plead your case, and explain that Momo only created the artwork based on the specifications of someone else.
These are derivative works based on preexisting characters, but they were drawn based on the designs of someone other than Momo. She has no legal claim here, and I encourage you all to fight back against this bully.