Use the adjective pretentious as a way to criticize people who try to act like they are more important or knowledgeable than they really are.
You might not be surprised to learn that pretentious is related to the word pretend, and it is an adjective that fits the bill for describing someone who's only concerned with making an impressive appearance. If you don't want to be accused of being pretentious, just act naturally and don't put on airs!