The way that you "come across" to people is how they see you and think about you. This phrase is a way to explain what people think of you, which can be different from how you think of yourself. So although you may think of yourself as cool and sophisticated, you might come across as unfriendly instead.
You can follow "come across as" with a noun phrase that describes a person:
He came across as a really friendly and sweet guy at first.
Or you can follow it with an adjective phrase that describes someone's personality, like in the example above. Here's another example:
Do you think I come across to people as kind of boring?
But it sounds strange to just have a single-word adjective after "come across as", like "come across as boring" or "come across as needy".