I hire people, I am a manager of a retail store, and I own a media company. I do take into account someones appearance, it's a clue to how they would fit in to the company environment. So it would come down to the company that you were applying for.
Well without knowing what exactly you're wearing to interview, I'd say perhaps its possible. Typically professional interview gear for women is nice dress shirt, usually white, dark suit jacket and matching skirt to the knee or dress slacks, and heels. Maybe you're mom feels its too conservative for where you are applying. If you're going to a job interview at say a surf hut on the beach or for a retail job, you can just go sans the suit jacket and go for a slightly more casual skirt/slack.
Depends on your outfit, and what kind of job you want. Over conservative is great for professional jobs, but if you're just going for a waitress then yeah you don't have to be conservative. What exactly were you thinking?