The Kano Chronicle (in Palmer's translation) mentions her and her conquests in the context of tribute in kola nuts and eunuchs. She probably did exist, lived sometime in the 1400s, and must have contributed to the growing importance of the trade in kola, slaves, and horses in Hausaland. It is also interesting to note that the Kano Chronicle references Amina of Zaria at the same time as Dagachi, a Borno prince who came to Kano in the 1400s. According to the Diwan, Dagachi was probably Uthman ibn Dawud, who reigned for a year (1421-1422) in Borno before being deposed. The Borno sources tell us that he died in Kano, making him likely the same person as the Dagachi of the Kano Chronicle. If the Kano Chronicle and the Diwan are reliable on the chronology, then Amina of Zaria must have been conquering and demanding tribute during the 1400s.
On the other hand, other sources suggest Amina lived during the 1500s, which seems less probable to me. The Emirates of Northern Nigeria: A Preliminary Survey of Their Historical Traditions by Kirk-Greene and Hogben claim she came to the throne in 1576, but that seems far too late and is harder to reconcile with other events in the region during the late 1500s.
Sources
Lange, Dierk. Le Dīwān Des Sultans Du (Kānem-)Bornū: Chronologie Et Histoire D'un Royaume Africain (de La Fin Du Xe Siècle Jusqu'à 1808). Wiesbaden: F. Steiner, 1977.
Palmer, H. R. “The Kano Chronicle.” The Journal of the Royal Anthropological Institute of Great Britain and Ireland 38 (1908): 58–98. https://doi.org/10.2307/2843130.