It's not. There's a movement that seeks to have it defined as dogma, but it has not been defined as such and it's most unlikely that it will be.
Although the Scriptures are crystal clear that Mary cooperated in the Redemption in a unique way, and that's all "co-Redemptrix" means. I suppose if we really wanted to get rid of it, we could just edit out the first chapter of Luke, and that uncomfortable stuff in Hebrews about Jesus being "born of a woman", etc.
UNMITIGATED DAFFYNITIONARY NONSENSE.