Yes.
In the description for Wither (click on the little question mark on the hero card to read the full details) it says that Wither is LESS EFFECTIVE on titans and monsters. Less effective means it still does something.
Test driving Boots on mobs (link here), he did exactly the same attack and def cuts to the S1 8-7 mob bosses as he would to a hero so at this moment it appears that vs. monsters is is the same in spite of what the card says.
You can always “burn” a titan flag and test it.