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!
adj making claim to or creating an appearance of (often undeserved) importance or distinction
showily imitative of art or artists
pretentiously artistic; cloyingly charming
grandiloquent, overblown, pompous, pontifical, portentous
puffed up with vanity
grandiose, hifalutin, highfalutin, highfaluting, hoity-toity, la-di-da
high-flown, high-sounding, inflated
pretentious (especially with regard to language or ideals)
(British informal) upstart
nouveau-riche, parvenu, parvenue, upstart
characteristic of someone who has risen economically or socially but lacks the social skills appropriate for this new position
abounding in or given to pompous or aphoristic moralizing
given to the overuse of long words
lacking aesthetic or social taste