dnd 5e – is the requirement of being in a lightly obscured area to gain the benefit of the Nature’s Mantle works in a heavily obscured area?

You don’t have to hide if you are heavily obscured.

Taking the hide action allows you to be unseen:

When you take the Hide action, you make a Dexterity (Stealth) check in an attempt to hide, following the rules for hiding. If you succeed, you gain certain benefits, as described in the “Unseen Attackers and Targets” section later in this section.

These benefits are:

When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target’s location correctly.

When a creature can’t see you, you have advantage on attack rolls against it.

So hiding lets you be unseen. But if you are in a heavily obscured area, you are already unseen:

A heavily obscured area–such as darkness, opaque fog, or dense foliage–blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

You don’t have to hide, you are already hidden.