There's more to magic than cards.
The first thing I would say is that having a hobby will often help one excel in, for lack of a better term, 'normal' life. People who focus 100% on school or work are not healthy and generally not very happy. They also have a tendency to be overly focused an unable to innovate. Whereas those with hobbies have other things to occupy their mind, which allows their brain to process things differently and approach problems differently, which allows them to succeed in more general terms. Not to mention that if you do have a day job, then any money you get through performance is just a bonus.
Also, having unusual hobbies makes you stand out to employers, which can help you get a job if your qualifications are similar to someone else who is also applying. My girlfriend loves to talk about a friend of hers who got a job because he's really good at whistling.
Performances, by necessity, teaches one to be calm under pressure and handle large crowds with ease. Like meetings and presentations.
I think what's really going on here is that she cannot see how you could make a living with those cards, so she assumes you can't. So either show her how you plan to do so, or show her that it won't interfere with your ability to do so.
This also kind of depends on where you are in life. If you're broke, unemployed, and not really making any progress toward making a living from performance, then she's right in a way. Something needs to change.
I think there is an association between cards and gambling, and criminals are often seen as stupid (unless they are the mastermind). However, the association she may be making may be entirely different. That would be that magic is stupid, which is one that is held by many. This association stems from how many truly bad magicians there are in the world, forcing their terrible performances on unwilling audiences.