It helps, but against healers that leech health (not Mundo) you'll really see the impact if you build deaths dance on top of that if your champion is ad. It's interaction converts part of their damage to your items damage which means they're not healing off that portion of their damage I believe.
Either way, Mundo is a bit too strong against it imo, but it's always worth it.
That said, if you see a life leech heavy champ in main or jg it may be a good idea to pick an ad that can use deaths dance well to counter it. (like urgot)
Leech heavy champs include kayn ult, brolaf, Warwick etc.)