If you look at https://fafsa.ed.gov/FAFSA/app/f4cForm , you can find that Pell grant eligibility extends to income levels that may be up to the bottom half of household incomes (though the amounts may be less than the maximum amount that Pell grants may give). So it obviously includes lower middle as well as lower income (but not the “$250,000 middle class” of these forums).
Realizing this means makes it obvious how strongly skewed the household income distribution at some colleges is. E.g. only about 10% of WUStL students have Pell grants ( https://nces.ed.gov/collegenavigator/?id=179867#finaid ), suggesting that only about 10% of WUStL students come from families in the bottom half of the household income distribution.