Yea, it stacks, butadditive, not multiplicative. So say a monster hits for 100 damage and you hit him 3 times with -30% damage debuffs, they do stack, and this debuffs him to 70 damage, then to ~50, then to ~35, removing 30% from his current damage each stack. If stuff stacked multiplicative he'd just get a flat -90% to his 100 damage and hit for 10, which would be broke as fuck. Same goes for bleed/blights, you can stack up multiple blights for 5 damage per stack, but a blight fo 5 and a blight of 5 isn't gonna multiply to a blight of 25.
Same is true for buffs. If you do a bunch of camp buff stacking for +damage, combined with trinkets and get your Hellion to +100% damage, she doesn't just do double damage, she does way more than that. It adds them in whatever buff % they are, so if she starts at 100 and gets +20% damage from 3 different places, she goes to like 100 base > 120 > 144 > 172, rather than just going to 160.