Grammar is a bit off but the dialogue is rather good. It's very close to what the characters would actually say.

I actually legit feel really bad for Zim because his entire life is a huge lie, that was made just to mess with him, and no one cares at all about him.

I just started this and it is already amazing! My favorite parts are the funny parts, like...

McGucket then noticed Dib. "That's one small head you got there, kid?"

"My heads not B-…. Uhhhhh. I'm kind of stuck on that one."

and

“Why thank you, Dib rip-off."

“Hey!”

El Profesor Membrana está en Gravity Falls.
-
Professor Membrane is in Gravity Falls.

Lindo capitulo.
-
Nice chapter.

Great story

La posición de amor causó muchos problemas.
-
The love potion caused many problems.

Pobre vampiro, jaja.

Ahora Mabel está interesada en Dib.
-
Poor vampire, haha.

Now Mabel is interested in Dib.

Parece que Zim va a recibir una paliza.
-
It seems that Zim is going to get beat up.

McGucket tiene un chupacabra como mascota.

Pobre Mabel. Ella se siente reemplazada.
-
McGucket has a chupacabra as a pet.

Poor Mabel. She feels replaced.

Gracioso.
-
Funny.

Zim no es tan listo como cree.

Mabel ama a los vampiros.
-
Zim is not as smart as he thinks.

Mabel loves vampires.

GIR y Mabel son amigos.
-
GIR and Mabel are friends.

Stan no quiere ser demandado otra vez, jaja.
-
Stan doesn't want to get sued again, haha.

So it ends this was a nice story. Glad I found it in a period before it was finished. Good job man.