Some people like, some people don't. Japanese food is not in the top of the Spaniards' foreign food preferences, but not in the bottom, either.
The two foreign cuisines most appreciated in Spain are Italian and Chinese, for sure. A new trend has appeared in the last two years and it's the Oriental restaurants, where you can find a bit of Chinese, a bit of Japanese, a bit of Vietnamese, a bit of Thai, and a bit of Korean, sometimes merged with Mediterranean food. The French cuisine is well valorated, too, but those restaurants use to be pretty expensive, what puts them apart from being too popular.