Frost resist effect from the mead and from a cape / wolf armor are all the same ; 50%.
Poison resist from mead is 75% whereas root armor provides 50%. Only the higher applies.

So it is worth taking a poison mead if you plan to encounter poison enemies, even when using root armor. But frost mead is useful only for corpse runs once you get a wolf or lox cape equiped. Mead provides no extra damage reduction against Moder or drakes attacks, even when wet (unless it all changed recently).