Good question. There's not a right of way according to the Ceredigion RoW map, but OSM calls it "permissive" anyway.

I noticed an information board with cycling routes as I was leaving, but didn't take a picture, sadly.

The "Visit Mid Wales" website lists "mountain biking" as an activity in the area, and says you can walk around the lake. There are no maps with details.

This is also on the route of the GB Divide. Hardly an official endorsement, but we certainly weren't the first to use it!

For what it's worth, I didn't see any signs indicating restricted access.