Not entirely because if the lender can bundle these bad mortgages together and sell them it's not really their problem if the borrower can't pay because they've already extracted a lot of value from the loan by selling it. Being able to sell the debt off to another party removes a lot of the issues a lender faces with bad loans.

Now in theory they still want to make all good loans but actually evaluating all the loans in a package is hard to impossible. This can incentivize lenders fudging the numbers on loans because a few bad loans failing is fine (so long as lenders as a whole aren't systematically issuing bad loans of course).