Was the letter to Mota a pivotal moment in The Trickster of Seville?

In terms of the letter being important to the overall outcome of the story, yes. In the story, the king of Seville has arranged for Don Juan to marry Doña Ana, the daughter of a commander. Doña Ana, however, is in love with someone else. Hoping to meet her cousin/lover, the Marqués de la Mota, that night, Doña Ana sends Mota a letter, telling him to meet her at 11:00 p.m. and wear a colored cape.
Don Juan intercepts this letter, disguises himself as the Marqués, and meets Doña Ana himself. When she sees that it is not the man she loves, however, Don Juan's entire plan collapses. Doña Ana screams, bringing her father running, and he and Don Juan enter into a duel. After killing the commander, Don Juan must leave the palace and heads to a small village, where he continues his own demise for the rest of the story.
If you look at it from this perspective, the letter to de la Mota was definitely a pivotal moment, because it led to Don Juan's eventual downfall. However, it could also be argued that the pivotal point here was the battle with Doña Ana's father instead, or even the killing of Doña Ana's father, as these are the things that Don Juan's downfall hinges upon.