I don't think you want to be working at the same company your mom does. From the sounds of it, there's no way that she's going to be willing to treat you as a co-worker rather than her kid. Plus, a company that shares application information with the applicants' mother is not a company that you want to be working for - that indicates a basic lack of understanding of confidentiality and appropriate behaviour. There's a non trivial chance that if you were hired you'd end up with your performance review being more like a high school parent teacher interview than a professional evaluation. But I agree with PP that the word idiot is coming from your mom's mouth, not the employer.
On the larger scale, though is the issue of your mother, which is really two issues. One is the "I'm not being rude/mean, I'm just honest" excuse for being an unpleasant and nasty person. The other is the fact that she's your mother, so it's hard to disengage from her behaviour or fight back.
In the first case, I find that the easiest way to tell if someone is direct vs nasty is to see whether they can take what they dish out. If your mother is the kind of person who can be told that's she's an idiot for doing something, or called stupid for making a mistake, without being insulted or getting mad, then you can probably get away with responding directly. I have a few friends like this - they're blunt spoken but can take it in return, and if I tell them that they've crossed a line, they'll listen, and we get along fine.
With the latter type (nasty person making an excuse), I've found the best options, depending on circumstances, are to either cut back on my interactions with that person, try to train them (leaving when they get nasty, only responding when they are reasonable, calling them on their behavior, as appropriate), or to simply totally ignore what they are saying and stop caring what they say, if it's someone I can't get away from or talk back to.
In your case, it's a much more complicated problem because it's your mother.
You don't say if you're living at home or not, or whether you're financially dependent on your parents. If you are, your first line of defense is to get some distance. Financial independence and your own place gives you a foundation for fighting back and setting boundaries in a way you can't do when you rely on your problem person for shelter and support.
The second line of defense is some good counselling. Breaking the patterns of years of bad treatment by a family member is something that can be very, very difficult to do, in part because after years of this it's hard to even recognize when behaviour is inappropriate, and even harder to stand up to it, when you've been trained to sit there and take it (or ignore it). Counselling with someone who specializing in family issues can help a lot.
A third tactic is to pull back on the amount of information you share with your mother. Don't tell her where you are applying for jobs until you get one. Don't show her your application forms - have someone else proofread or critique them for you. Find other people to share your triumphs and disappointments with - friends, other relatives, but people you can trust to be supportive rather than cutting you down