Morrita is a colloquial term in Mexican Spanish for a young woman or girl. It's similar to saying "chick" or "girl" in English, often used in a casual or affectionate context.
In this song, Natanael Cano uses "morrita" to refer to the young woman who is calling him, setting a very informal and contemporary tone. It's a word that gives insight into modern Mexican slang and is frequently heard in regional Mexican music.
Natanael Cano basically lays out the whole Corrido Tumbado lifestyle in this song. It's all about a certain kind of success that looks like one thing from the outside and feels like another from the inside. He complains that people see his flashy style, like being "adiamantado hasta los dientes" (diamond-studded to the teeth), and immediately assume he's a "delincuente" (delinquent). He's surrounded by luxury, like the "Dos Cayennes" (Two Porsche Cayennes) that follow him, but also by danger, hinted at when a girl hugs him and "siente la glocksita" (feels the little Glock).
There's also a whole secret relationship happening. He's in his Mercedes, playing his own music, with a girl who has to turn the volume down because "Su vato piensa que yo soy su amigo" (Her man thinks I'm his friend). It's a song full of boasts about being on top, but it's also about the constant need to watch your back and navigate a world where appearances mean everything.