Here is the NC statute for Felony Death by Vehicle
There are two levels of it, as you can see, an Aggravated Felony death (a class D felony) and Felony Death, which is a Class E felony. The penalty range for a NC Class E felony is 15 to 31 months of incarceration. If you look at it that way, the sentence that your wife got appears generous.
The only way she could have gotten so low a sentence is on a negotiated plea bargain where the DA came off of the top count and let her plead guilty to a misdemeanor charge with a reduced sentence. Sometimes that's done just to spare the feelings of the bereaved family who would otherwise have to endure a trial. It doesn't necessarily mean that the DA couldn't get a felony conviction if your wife wanted to go to trial on the case.
It is very difficult to get a plea back once a defendant has already pled guilty and been sentenced. That's because judges are always careful to do all they can to make these pleas airtight. They ask the defendant a lot of questions about whether she knows what she's doing, whether she's happy with the advice, whether she knows the rights she's giving up, whether she understands the terms of the promised plea, whether she's been in any way forced into taking the plea and so on. All of that is taken down by a court reporter, and it will be used against your wife when she tries to get out of the plea. If she goes back before the judge on her sentencing day and says that she has changed her mind, the judge does not have to let her get the plea back, and they usually would not. You would then have to hire a post-conviction lawyer and appeal it. With only 90 days in jail, she'll be out long before the appeal is ready.
Another thing you and your wife should understand is that if the judge does give your wife the plea back, the felony charges will be restored to what they were, and she will not get another misdemeanor deal from the DA. The judge will not be able to give her less than 15 months. So that means she will have to go to trial, where she would be risking up to 31 months if she is convicted on the felony. She and you should consider the risks very carefully before she gets her plea back. She's not likely to do better than 90 days in jail when there was a death in the case, and she could definitely do a lot worse.
But if she decides to try to get her plea back she should call her lawyer and let her know that on the sentencing date she wants to ask for the plea back so that if there's any good legal argument to make to get her what she wants, her lawyer can make it.