No, it is not good to feed your dragon spinach. Spinach gets its own article because it is a commonly found green that you might consider feeding your dragon if you didn't know that it was unhealthy for 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. While a bite won't harm them, this can be said for many things that aren't safe for your dragon to eat, and it is better to just be avoided altogether.