It is normal and not a problem. It’s rather unfortunate that these indicators in luci don’t show what people want to see. There have been multiple threads on this, e.g. Too much RAM eaten
It doesn’t seem you really understand. Luci doesn’t show “available memory” statistics, which is what people naturally consider as free memory. It’s most likely that your memory is just fine; it’s just not possible to determine that from that page alone.
From tools available in Turris packages, I’d probably use htop, from inside ssh to the router.