What is salsa criolla campesina of Nicaragua?
I haven't found a specific recipe so far - but I did find this:

'Criolla de Cocina' is a unique chiltoma pepper from Nicaragua. It is a sweet pepper characterized by a strong flavor, thin walls, and wrinkled appearance (like an oversized habañero). These types of peppers are used in Nicaragua for the classic sauce known as salsa criolla campesina, which combines thinly sliced onions and peppers with tomatoes, salt, sugar, and vinegar, and is served with grilled or fried fish or chicken throughout Nicaragua.

from here.
Of note is that campesina translates to peasant and criolla to creole or native in english, so you might translate the name as the sauce made by the native people (says me with no Spanish beyond a few basic words) - so it is likely a regional dish without a defined recipe - a true dish of the people if you will.
However: look what I just found here: http://www.recetasnica.com/Salsa-Criolla.php. Translated below:
Creole sauce .
Ingredients:

2 Large Tomatoes, Finely Chopped
1/4 Cup of Bitter Orange or Lemon Juice.
1/8 cup of vinegar.
1/4 of Cup of Water.
2 Fine Chopped Chiltomas.
2 finely chopped onions.
6 Fine Chopped Jalapeño Peppers
2 Tablespoons Ketchup (Tomato sauce).
1/2 cup of finely chopped coriander.`

Procedure:
Mix all these ingredients and add salt, pepper and hot sauce to taste (you can  also make it without chili). Put it in a separate container. This sauce can be added to any type of food or soup. You can keep it in the refrigerator indefinitely.