Hmm, there is no united conformation on the origins of the owlbear(in lore, in real life we know Gygax based it on a toy). Dragons and other monsters are also magical creatures but still considered to be parts of nature in Faerûn. So a druid being against its existence just based on that feels wrong.
No, Owlbears are not magical beasts - they are, quite literally, Monstrosities. It's a creature type, with specific meaning and implications.
From the Monster Manual:
Monstrosities are monsters in the strictest sense — frightening creatures that are not ordinary, not truly natural, and almost never benign. Some are the results of magical experimentation gone awry (such as owlbears), and others are the product of terrible curses (including minotaurs and yuan-ti). They defy categorization, and in some sense serve as a catch-all category for creatures that don’t fit into any other type.
They aren't natural or part of any natural order.
