Can a borrower take legal action against the lender if they were improperly qualified for a mortgage?

IF the lender falsified your information (e.g. income, assets, debts) without your knowledge or consent to give you the loan, you may have a cause of action against them (though after 6 years, you may have exceed the time period to sue, called the statute of limitations).

However, if by "improperly qualified," you mean merely that it was probably a bad idea to qualify you, but all the information was accurate--or it was inaccurate, but you signed off on the inaccurate information--then you would have no cause of action. In the former case, the lender did nothing legally wrong (the law does not require banks--or anyone--to make good decisions; just to not deceive, defraud, etc.). In the latter case, your own wrong doing--signing off on incorrect information--would preclude you from recovery.