I think people who make $150k+/year can declare themselves middle income for college financial aid if that’s what they want. But then they also have to accept that the people who make ~$125k or less, and the people calculating need-based grants, get to determine their definitions as well. I don’t think claiming that you’re a middle income family is going to move many colleges to offer your child need-based aid.
If colleges choose to give need-based aid to only those making ~$125k or less/year then, in my opinion (based on the definitions here), they’re granting that aid to only low income families. If they were granting need-based aid to middle income families then those making ~$150k or more per year would be getting some, but that doesn’t appear to be the case for many. Families who don’t get enough aid have to look at less expensive schools.
I don’t believe that middle income families are shut out of expensive colleges any more than low income families are. If anything, they’re more likely to be able to send their kids away to college than low income families. They may not be able to pay ~$65k/year out-of-pocket, but many who post on CC seem to be able to afford ~$30k; some can even pay as much as ~$45k. The typical low income family (for this purpose those making ~$125k or less) with an average stats kid is going to be hard pressed to afford the colleges that families making upward of $150k/year can afford.
So feel free to use whatever definition you want, just don’t make the mistake of believing that it can (or should) qualify you for need-based aid. Run each college’s Net Price Calculator and make sure your child applies to a financial safety.