Of course you can, I mean, you are able to ask the question, but sometimes you shouldn't. Some people could think that you are a meddler.
I don't think you should ask for anyone's income unless you knew a lot this person. I don't recommend you to ask for incomes, specially to foreign people whose customs could be very different from yours.
Although there are some cases in them asking for the income would be understandable, for instance if you are going to rent a flat to someone, because you would need to be sure that this person is going to be able to afford the rent.
I hope my opinion will help you.