Maruko is complaining that mum is buying seaweed as a new year's present. Mum replies:
「のりは食べる機会が多くてすぐに減るからもらったらありがたいもんなのよ。」
There are plenty of opportunities to eat seaweed, and because they immediately decrease, when one receives it they are grateful.
Why do the opportunities to eat seaweed decrease? Is there some cultural knowledge I'm missing or is my translation wrong?