It’s not necessarily fine. An open mouth can be an indication they are too hot. Chameleons do not sweat so it’s a way of cooling off. As a possible offshoot of this, he may also be getting burned if his basking area is too hot. Verify that your temperatures are correct just to be on the safe side. Cesca, your chameleon looks great (healthy, no burns or dry skin) but I don’t want you to have problems down the road.
We have written a great resource for new owners and it covers everything you need to keep a chameleon healthy. Check it out!
https://www.chameleonforums.com/care/