Tax isn't a "punishment", it's an important part of building infrastructure among other things.
Yes, poor and middle class folks may not have the financial knowledge of wealthier individuals, but doesn't make a flax tax ok. It would put the burden on poor and middle class folks who will definitely need that 10% more than someone who's making way beyond their needs.
It would also cause a massive decrease in revenue for the government, which would do long term damage to infrastructure, social programs, etc. Harming the middle and lower class even more.
It's not a good idea.