As far as I remember the fertile lands of Mordor around lake Nurnen were given by Aragorn to the freed slaves of Sauron, so I guess this part was fertile and habitable.
As for Northern Mordor, Gorgoroth etc, it seems heavily polluted by Sauron & co and low on rainfall, added to that Mt. Doom blew ts top and preumably covered the place in ash, pyroclastic flows, lava, spikey rocks, bits of winged beast etc. The Towers of the Teeth collapsed at the destruction of the ring but don't know about Cirith Ungol. Certainly Aragorn intended to destroy Minas Morgul,
Quote:
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'For', said he, 'Minas Ithil in Morgul Vale shall be utterly destroyed, and though it may in time to come be made clean no man may dwell there for many long years.'
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I'd guess that that must of been true of most of the nastier bits of Mordor.
However, to speculate, if the climate had improved and Northern Mordor got a bit more rain it would be likely that plants would return swiftly and recolonise the place. For example, the area around Mount St Helens in the USA was devastated when it blew up but is now recovering, the remaining bits of Krakatoa are now lush jungle etc.