You only need swap (or fly scale vm memory 512 to a larger 512MB instance) if your instance keeps getting killed with OutOfMemory errors.
If not, as the post which I linked to explains, it apparently is pretty normal for NodeJS to reserve as much free memory as it can, upfront.
ssh into one of the app instances fly ssh console -s -a <app-name and exec free -h. That should show you available RAM including swap, if any. I don’t believe swap is accounted for in the metrics that Fly collects, which you see on the dashboard.