I get that voice-over in animation has far looser restrictions for available roles, but like >>115336205
pointed out, there just aren't that many black female VAs with major roles even today. I feel like that's partly due to most of them being reduced strictly to "black-sounding roles," whereas someone like Kristen Bell might have more options.
It also has to do with so few cartoons starring a black female character as part of its main or recurring cast. Molly Tillerman was a good window for many black actresses to have made their break, so it feels a little gross to have Kristen Bell just strip them of that chance through nepotism and star power.
But again, I don't mind it too much so long as the actress who won the role in the end still does a great job, which Bell does. Just like I can't be too bothered by John Roberts voicing Linda since he does such a great job at it. We could just stand to use more black [female] leads. It's not required, but I definitely welcome it.