What makes you think God disapproves of prayer intended to ask His mercy for someone else?
Nothing at all, we are called to, but upon death, the die is cast. Up or down. If you have Jesus as your Savior, you already have His mercy, if not, well, there ya go, and no amount of prayers, hail marys, or begging is gonna get you there.
As for answering a question with a question, you still have not answered mine. WHy is Jesus not sufficient for you to go to heaven, to get there? Why do you feel His sacrifice ‘needs a little extra help”?