Me - I hate washing dishes. But someone has to do it and at age 18 I got a job which involved some days washing dishes for hours at a time. Is it worth it? Its probably good for you to do work like this. You can work out your own system, find the problems and solve them, keep your brain active, stop being embarrassed at asking for help. If you can show that you can do this, full of enthusiasm, then it will be noticed and you will likely get to do more interesting things - but that is only with their agreement. And this isn't going to improve your flying. You should be going into this with your eyes open.
As I said in a previous thread, as you are doing something for a reward - its technically not volunteering and so you should be being paid at least minimum wage. If you aren't being paid minimum wage, you are being exploited.
See
Volunteering England - Volunteering England for volunteer agreements, but I'd be surprised if they have one of these, as they are really a statement of policies as they can't look like a contract. If its a contract - you need paid.
Only you can decide whether its worth it. See if you can get agreement to do other things when its quiet, but it sounds like you have just taken in a big gulp of the real world!
Note - you are unlikely to be allowed to serve or make food as you wont have a food hygiene certificate, and as they don't know anything about you, they aren't going to give you access to the till.