By a Yoga mat, I assume she means the roll-up close cell foam ones.
Much like the old-style roll-up flat camping mattresses pads, like https://www.campmor.com/c/coleman-rest-easy-camp-pad
made of "Closed cell polyethylene foam", or other closed cell foam, which almost all backpackers used to use, but which have been largely replaced by less bulky (when packed) but heavier and more expensive alternatives. Though 3/8" is thicker than you need - you'd have to cut it down - as I do with carpet foam. The way I see it, you want to cut it (on the bottom) to match the shape of your foot anyway.
The carpet foam padding I looked at is much the same thing, but cheaper. However, I just checked, and there are several different types of carpet padding - some might not be good for your purpose. I only looked at the most expensive stuff, with high "rebound".
Any of these foams would make great sew-in knee and elbow pads too...
If your feet get cold while skating, these closed cell foams are much warmer (maybe 3-10 times higher R-value) than the insoles most boot makers provide. If your feet get hot and sweaty, an open cell foam might work better for you.
Some of the really cheap Yoga mats, like https://www.amazon.com/Kabalo-PURPLE-Non-Slip-Exercise-Camping/dp/B00VPMSB80/ref=sr_1_26?s=exercise-and-fitness&ie=UTF8&qid=1489088727&sr=1-26&keywords=yoga+mat https://www.fivebelow.com/sports/yoga-mats-and-fitness-balls/solid-blue-yoga-mat.html
are probably thinner. They might be good enough, especially if you are willing to glue layers together. However, some of them are of a higher density foam, that won't be as warm, and probably won't cushion you as much. But it's your choice. You can certainly experiment.
Because I'm not much of a jumper, and not a great skater in general, I don't know whether you can have too much padding for good control.
A rep for one boot maker rep (from Avanta?) told me that a closed cell foam insole would be considered an energy storage device, i.e., a mechanical aid, and would therefore be against skating organization rules for tests and competitions - but I haven't been able to find any rules that forbid mechanical aids. AFAICT (I'm not an expert!), you could embed springs in your skates
to jump higher, though control might be a problem.
I have a web page (see my signature) on modifying boots, including a discussion of making your own insoles - but the focus is on improving fit, not padding.