Yes, I believe some mortal sins are worse than others. However, it is a rather complicated problem.
First of all, it can often be very difficult to compare one sin to another and decide which is objectively worse. In two related sins, like normal pornography vs. child pornography, we may easily be able to discern which is worse. But what about adultery vs. theft? And remember also that all these categories are generally very broad, including a wide variety of different possibilities. Exactly what kind of act does the pornography show, for example, or exactly what was stolen and from whom? These sorts of things can affect the sin's level of gravity, including its relative gravity compared to something else.
Then there is the purely subjective side of the sin. One could make a case for masturbation to be a graver sin than fornication or for suicide to be a graver sin than the murder of another person in an objective sense, but on a personal, subjective level, bringing in feelings, knowledge, consent, deliberate choice of evil, etc., it can be the opposite.