It's not necessarily a sign that the cage is too small. In fact, it's not necessarily a sign of anything wrong at all.
Chameleons will use the mesh of the cage to climb on from time to time: sometimes it's just the simplest route for them to take when they want to move around.
However, if you find that your chameleon is spending more time on the mesh than on the perches and plants in the cage, it could be an indication that you haven't provided him with enough branches/vines to navigate his way around the cage. Make sure there are ample vertical and horizontal pathways for the chameleon from the bottom to the top of the cage. That should keep him off the mesh most of the time, but even then, he will probably wander onto the mesh occasionally.
If the chameleon is climbing on the mesh at the top of the cage, it could be an indication that it is looking for a warmer basking spot, and is trying to get closer to the lights. You could increase the temperature of the basking spot a bit to prevent this. But be careful not to burn the chameleon - don't make it too hot, or put the lights too close.
Finally, a common indicator that the chameleon is uncomfortable in the cage and wants to get out is when the chameleon goes to 'ground' in the cage. If you find the chameleon walking around on the floor of the cage a lot, and walking along the mesh at the bottom of the cage, it often means that it is looking for an exit. This could be because the cage is too small for it, or that something else is making it uncomfortable in the cage. It could also just be adolescent restlessness - males will often want to go exploring.
In your case though, climbing on the mesh once in a while is quite normal.