A* is as valid on a navmesh as it is in a grid-based layout. A* just needs "points" (or each edge of a navmesh polygon) and the edges that connect them, how those points and edges are represented or exist in the world is entirely an implementation detail.
It's funny cause A* (a slight modification of Dijkstra's algorithm) is explicitly an algorithm that operates on graphs. Applying it to a "navmesh" is actually conceptually simpler than thinking of a grid-based game world as a big uniform graph.
