Well, it is. The novel contains elements typical of romantic novels. For example, it explores intense emotions. Victor's obsession with creating life is an extreme form of passion. Also, the idea of the individual's relationship with the natural world is prominent. The Creature finds solace in nature at times, and the vast, unforgiving landscapes add to the mood. Moreover, the novel delves into the concept of the outcast, which was a theme in Romantic works, with the Creature being an ultimate outcast.
I think 'Frankenstein' can be seen as a romantic novel. One aspect is the focus on the power of the imagination. Victor's ability to conceive of creating life from dead body parts is a huge leap of imagination. This is in line with the Romantic emphasis on the creative power of the mind. Additionally, the sense of melancholy and longing that pervades the story, especially in the Creature's search for connection and meaning, has a romantic undertone. The novel also questions the limits of human knowledge and ambition, which was a common concern in Romantic literature.
Yes, in a way 'Frankenstein' can be considered a romantic novel. The relationship between Victor Frankenstein and his creation has elements of a dark, distorted form of love. The Creature longs for acceptance and love from his creator, much like a jilted lover. There are also the overarching themes of nature and the sublime which are common in Romantic literature. The desolate landscapes and the powerful forces of nature described in the novel are characteristic of the Romantic movement.