Yes. Frigo is a shortened form of frigorifero.
That said, it is extremely common and completely normal in everyday speech. It is not strange or incorrect. It is just less formal than frigorifero.
So:
Merenda means a light meal or snack, often eaten in the afternoon. It is especially associated with children, but adults use the word too.
It is close to snack, but it can also suggest a regular snack time, not just any small thing to eat at any moment. So it often has a slightly more specific feel than the English word snack.
Here per means for in the sense of intended for, meant for, or saved for.
So è per la merenda means the pineapple is intended for the snack time.
The article la makes it sound like a specific snack occasion, such as the expected afternoon snack. You may also hear per merenda, which is a bit more general.