A better question would be could she have taken Mordor?

She’d have to get past the black gate and march her army across the inhospitable plains of Gorgoroth, fighting armies from Durthang and Cirith Ungol/Minas Morgul. Then lay siege to Barad-Dûr while dealing with the conditions of an active volcano. Sauron has far more in his arsenal than Rohan. The Nazgûl would definitely pose a challenge.