The sentence you wrote is: "The pattern on a giraffe’s skin is brown spots."
This sentence is mostly correct! It clearly tells us what the giraffe’s skin looks like.
If you want to make it sound even better, you can say: "The pattern on a giraffe’s skin has brown spots." or "The pattern on a giraffe’s skin is made of brown spots."
Both ways explain that the giraffe’s skin has many brown spots as its pattern. But your original sentence is simple and easy to understand, so it’s good for now!