In short, it won't work because first, wood expands and contracts significantly with moisture and temperature changes. In an outdoor patio or porch, these changes are significant and would cause cracks to form at the joints. Most plywood ceilings are finished with a fir or other finish veneer that can be attractively stained or painted. For a more traditional look, you could also nail an attractive V-groove or bead board panel over the top of a rougher grade of plywood if that is what you have.