No, it isn't. It is the Present Active Simple Tense of the verb 'to be'. The word 'served' is the Past Participle od the verb 'to serve'. Пасха является [is ->to be] традиционным сладким блюдом, подаваемым [served] в России, так же, как и в других восточноевропейских странах.

Yes. "served" is the abbreviated form here for "which is served" A past participle which comes after a noun is never a participial adjective. Participial adjectives appear before the noun, like other adjectives.