mendacity is a great word for untruthfulness (though 'untruthfulness' is another possibility):

The quality of being mendacious; untruthfulness; tendency to lie.

*On a side note, Tennessee Williams used this word several times in Cat on a Hot Tin Roof. Here's one scene.

mendacity is a great word for untruthfulness (though 'untruthfulness' is another possibility):

The quality of being mendacious; untruthfulness; tendency to lie.

You could say:

I care about mendacity.

That would mean that you care about untruthfulness.