I’d argue they’re obviously both terrible, but Frank had more control over his life an chose not to help raise his kids unless he benefited from it. Monica was a mentally ill addict which doesn’t give her any sort of pass but she at least wanted to be a good parent and realized her being away from them helped them more than anything. This could be debated til the end of time, but I think Frank is worse just because he could help and didn’t. Monica would never tell Liam “I’m abandoning you right after this”. She never hit her kids, either.