Is there a way in netsuite to assign seasonality to products?

From a demand planning standpoint? Yes. On an item record under Purchasing/Inventory > Inventory Management, you can find these fields

We make a custom field sourced from a custom list with our different seasons as well (Summer, Winter, Holiday...) Since we don't use the built-in demand planning, it works for our product/sales teams to sort/review.

If your talking about just labeling products with seasons for reports then a few custom fields work well like @Mark C said If you want some automation to inactivate/activate products based on their season then that would require a customization Depending on its complexity could be done with a workflow but might need a script