I would say that because you were intending to look at nude pictures of other people for your personal gain (arousal), then that amounts to objectifying those people and selfishly using them without regard for their dignity as children of God. This is a major part of why things like pornography are wrong: Because they not only take the sexual act outside of it’s intended purpose, but because they also diminish the human person to a mere object to be used selfishly.
Now, that may not have been your line of thinking, and you may not have seen it that clearly in the moment, so your culpability may be diminished. You may also have an inordinate attachment to this type of sin.
Best to talk over the specifics of your situation with a priest in the confessional to determine whether you’re in a state of mortal sin… But as a rule of thumb the fact that you have to question if you are or are not would suggest that you probably are not.