Is it stupid to use abbreviations like idk an lol?

[Minjung]

it depends of the circumstances. On a serious writing like at school or whatever, then there is nothing more dumb than using ''lol'' or ''idk''. But when you chat with friends or comment something on websites such as facebook, youtube or tumblr, it's generally informal so it's alright.
Also, if you make a facebook or twitter status, avoid it.
it's really more for conversation purposes than anything.
Every single person i know uses them so i can confirm you it's definitively not stupid.
I don't know how old you and your friends are but just learn to use it when its the right time and now one should ever make fun of you because of that.