Not as a "staple" food, but it is okay to offer a leaf or two every once in a while.
Spinach gets its own article because it is a commonly found green that you might consider feeding your dragon. Other fresh greens such as kale, mustard greens, and turnip greens are great for your dragon to eat as a staple. But spinach is very high in oxalic acid; this is a chemical that inhibits calcium absorption, which means it will stop your dragon from absorbing the calcium it eats.
Too much oxalic acid and this will lead to a calcium deficiency in your pet, which, if left untreated, will progress into metabolic bone disease or MBD. MBD is a severe condition that can kill or severely deform your dragon by weakening their bone structure.
Spinach is, however, a rich source of iodine, which help counteract other greens that are high in goitrogens. Plus, it increases the variety in your bearded dragon's diet. So it can actually be beneficial to offer spinach leaves once every two weeks or so.
"Spinach" by Cool Librarian Photographer is licensed under CC BY-NC-ND 2.0
