Hola Harsh

It can use both the imperfect and the preterite. In this case, there is a previous clause saying "Yo lo vi en Sevilla" so the next clause "y fue maravilloso" is linked to that previous clause and continues in the same tense. The speaker is using the preterite because the "being wonderful" is seen as something that happened and is considered to have an end, as opposed to the imperfect "y era maravilloso" which, as you said, would have more of a descriptive purpose without considering that it had an end. 

With the preterite it's as if we put an end to that experience, i.e. it was wonderful during the time when I saw it. 

I hope it clarified it. It is a tricky topic. We have this lesson here specific for the verb ser in the imperfect and the preterite.

Saludos

Inma