I don't think it's ever too soon to get married. It is true that knowing someone longer can put the two people in more real-time situations that demonstrate a wider variety of behaviors, but people change into monsters for a variety of reasons, and sometimes they don't show any of the red flags at all until they are under the right circumstances.
Why would anyone think that it is too soon for Mariah Carey to get remarried? What one does with their lives is up to them. There is no manual that dictates when someone can get married. If she found someone she loves, and who loves her back, then all the more power to them.
I do not believe it is too soon for Mariah Carey to remarry. Every person leads a very different life and whatever situation feels best for him or her is what that person should do. As someone with young children and who is getting older, Mariah may feel that she not only needs or wants help raising children, but that time is too short to be spending it alone. There is no time limit on divorce or marriage and she should be able to remarry as she chooses.
Mariah Carey is an adult in a consenting relationship with another adult. She has handled her business with Nick Cannon, and what is done is done. If Carey feels she's ready to get married, there's no one that can stop her other than herself or her recently betrothed, and public opinion shouldn't play into their decision. Both of them have been married before, so this isn't a case of inexperience.