Does Le Chatelier's principle apply to the gas inside an air mattress?
No, the air we breathe is about 78% nitrogen, 21% oxygen, 0.5% carbon dioxide and other gases. Nitrogen doesn't react with anything at room temperature and there is nothing to burn inside the air mattress. 

What probably happened was that there is a very small hole that the air leaked out. About the only way to find the hole would be to submerge the air mattress in a pool.