Pendejadas is a colloquial and somewhat vulgar term that translates to "stupid things" or "nonsense." It's a strong word that conveys frustration and disdain.
In the context of the song, the singer uses it to express his anger and disbelief at the ungrateful and arrogant statements made by the person he invested so much in. It highlights the raw emotion and the sense of betrayal he feels, making it a very impactful and memorable word in the lyrics.
Voz de Mando's "Y Ahora Resulta" is a really blunt and funny take on a relationship gone wrong. The singer is absolutely fed up with someone they feel they completely transformed, only for that person to become arrogant. They list all the things they bought and changed about this person, from designer clothes and bags to plastic surgery like "te puse pechos, te puse nalgas" [I gave you breasts, I gave you butt cheeks] and a nose job. They even mention buying more shoes "que para un cien pies" [than for a centipede] and a car the person can't even drive!
It's clear the singer feels they invested a lot, only to be told they're "no estoy en el nivel que tú pensabas" [not on the level you thought]. The chorus, "Y ahora resulta que te sientes el más bello monumento" [And now it turns out you feel like the most beautiful monument], really drives home the singer's frustration and regret, calling the whole thing "una mala inversión" [a bad investment]. It's a pretty savage song, especially with lines like "Maldita puta, antes de mí, tú no eras nada" [Damn bitch, before me, you were nothing], showing just how angry and betrayed the singer feels.