This is a good question. That I am aware of most of the conquistador accounts don't mention this issue directly, which doesn't mean they didn't notice it. They just didn't write about it at any length.

The other issue involved is that during the conquest of Mexico, the Spaniards were aided by native warriors from groups that opposed the Aztec/Mexica Empire. Having those warriors on their side likely mitigated against the 'fear/dread' issue.

/u/historianLA is correct in that the Spanish accounts aren't particularly forthcoming on this direct question. There is a passage from Bernal Diaz del Castillo which seems pertinent though. This from the period when the Spanish were traveling overland from Cempohuallan and Veracruz towards Tenochtitlan, at which they had to pass through Tlaxcala, a rival Nahua state to the Aztecs. This would have been the first large battles the Spanish fought against "Aztec"-style troops:

We had scarcely proceeded a quarter of a mile when we found the fields covered with warriors; they had large feather-knots on their heads, waved their colours, and made a terrific noise with their horns and trumpets: indeed, the pen that would wish to describe everything we saw here, would not find it such an easy task! this was indeed a battle of as fearful and dubious an issue as well could be. In an instant we were surrounded on all sides by such vast numbers of Indians, that the plain, here six miles in breadth, seemed as if it contained but one vast body of the enemy, in the midst of which stood our small army of 400 men, the greater part wounded and knocked up with fatigue. We were also aware that the enemy had marched out to battle with the determination to spare none of us, excepting those who were to be sacrificed to their idols. When, therefore, the attack commenced, a real shower of arrows and stones was poured upon us; the whole ground was immediately covered with heaps of lances, whose points were provided with two edges, so very sharp that they pierced through every species of cuirass, and were particularly dangerous to the lower part of the body, which was in no way protected. They fell upon us like the very furies themselves, with the most horrible yells; we employed, however, our heavy guns, muskets, and crossbows, with so much effect, and received those who pressed eagerly upon us with such well-directed blows and thrusts, that considerable destruction was made among their ranks, nor did they allow us to approach so near to them as in the previous battle: our cavalry, in particular, showed great skill and bravery, so that they, next to the Almighty, were the principal means of saving us.

Despite the tumult presented, the passage is indicative of Diaz del Castillo's professional, even laconic style, where a fierce battle passes in a paragraph. Part of this was because he was a veteran of at least one previous expedition to Mexico, but also because he was writing several decades after the fact, giving time the chance to dull any sharp impressions. Keep in mind that the Spanish had the luxury of having a slower introduction to the Aztecs than vice versa, having moved first into the Caribbean, then scouted along the Yucatan and Gulf Coasts, before moving into the more central areas.