It could have been good, but only reached tolerable. It began with a weak premise. One of my least favorite things in any story is when everything could have been cleared up in one single, short conversation. If the writers have to use a plot-line that easily fixed, it's lazy.
Looking beyond that, Elodie (Millie Bobbie Brown) continually screaming as she is trying to run and hide from a dragon within the dragon's mountain. Very early on the dragon states that she can hear Elodie. So naturally it takes Elodie awhile to work on shutting her mouth as she is trying to save her own life. Towards the end she figures it out and out loud states something along the lines of "Be quiet, she can hear you." ... dumb main characters are no fun to follow or root for. Amd I should hope the audience would know that when we're trying to hide, we generally want to be quiet, not screaming/grunting/whimpering.
Furthermore, the characters were one-dimensional and easy to figure out.
What's positive about the movie? The visual effects.
I wanted to love this. In fact, it's the first Millie Bobbie Brown project I've been disappointed in. The acting was fine, for the script they were given. But nothing saves a bad plot with stupid characters.