Idealistic , with one's head in the clouds.

Antonym for "down to earth " would be "Proud". Usuage is sentence : 1- My elder brother is really down to earth but my youngest brother is somewhat proud . 2- My boss is a proud man but he pretends to be down to earth in front of his supervisors . Does this make sense ?

You first have to define „down to earth.“ That‘ sort of means honest, but not directly, and straightforward. So guess the opposite is superficial, and somewhat hard to connect with. But superficial is the best that comes to mind.

The opposite of "down to earth" is "high maintenance" ....among others!

Someone can think they are the shit. For example, “Andre thinks he’s the shit just because his father owns the company”.