I assume you are referring to the United States. There are a variety of laws and executive orders in the "fair housing" category (Wikipedia has a good list) which mostly cover protected classes. Although low-income is not a protected class, there is a risk that if the net effect of your change was to exclude members of protected classes, you might be risking a lawsuit. For example if you had a condo complex that was 30% minorities, and you raised the income requirement and saw the minority number drop to 0%, a plaintiff might be able to argue that you were violating the Fair Housing Act.
Long before you reach any legal limit, though, you are going to reach a practical limit. In many markets you will find that if you set the income requirement much higher than usual, you simply won't have tenants. And there seems to be a lot of evidence that income requirements are not effective at getting credit worthy tenants. For example, young families often need more bedrooms and might only have one parent working while the other parent takes care of the kids; these families spend a high percentage of their income on housing, out of necessity, but are relatively good credit risks because they are motivated to maintain a stable residence.
I recommend reading Minimum Income Requirements Can Be Another
Form of Housing Discrimination (PDF) before you proceed.