When in spectator mode, any clicks on the dragon have no effect. You don't spectate the dragon, and when dismounting (As if you were on it.) Also does nothing.

You can spectate the dragon with the command:

/spectate @e[type=minecraft:ender_dragon,limit=1]

However, when spectating the dragon, you are rotated the wrong way. You are facing the dragons rear end instead of its head.