When is it good to use "ダサい" (out of fashion, boring)? Is it an everyday word?
Is it an everyday word?
I think it certainly is used enough for it to be a popular parlace slang term most speakers will be familiar with.
When is it good to use "ダサい" (out of fashion, boring)?
It helps me think of it as an approximate translation of lame and how speakers use that word in English. There might be a slight caveat in this though, in that ダサい seems to be directed more at what is considered by the speaker lame fashion and not exactly attitude per se, as suggested by this answer in Chiebukuro asking about the difference between ダサい and カッコ悪い. So this would include "out of fashion".
For the general meaning of "boring" it is a harder fit in my opinion. I'd probably be using つまらない instead.