Accusing someone of cognitive dissonance is a way of saying you proved them wrong - you found some inconsistency in their thought they couldn’t resolve.

If instead they think that it’s ok to fuck a dog, you didn’t do that. You found someone who disagrees with you (and, I’m sure, the vast majority of people) which is not special. You should describe what you found - someone who thinks it’s ok to hurt animals - instead of accusing them of inconsistency.

It matters because disagreement doesn’t mean they’re wrong. That’s something else you’re taking on the task of convincing people of. You shouldn’t cheat by lying about their beliefs, even if you do think at least one of their beliefs is disgusting - you can just let other people conclude their beliefs are disgusting.