Is this an official Jarritos shirt?

It is sold as Jarritos-branded merchandise featuring the Harrytos character. Buy from a seller that states the licensing, since unofficial prints of the same artwork exist.

What is the shirt made of?

It is a cotton jersey T-shirt, the usual fabric for graphic tees. Cotton breathes well but can shrink in a hot wash or dryer.

How should I wash it to keep the print?

Turn it inside out, wash cold and tumble dry low or hang dry. Heat is what cracks and fades screen-printed graphics.

How does the fit run?

Graphic tees like this usually run true to size, but cuts vary between sellers. Compare the chest measurement against a tee you already own rather than guessing.

Is it worth buying over a plain tee?

Only if you want the Jarritos graphic. As a garment it is an ordinary cotton tee, so you are paying for the licensed artwork, not better fabric or construction.