Informal language may make use of slang and colloquialisms, employing the conventions of spoken language. However, it is too casual and loose to be acceptable for academic writing.
While writing a diary or scribbling letters to your close friends and while conversing with a close friend, using informal language is considered to be OK.
When used to mean "somewhat," kind of is informal: She's kind of cute. When used taxonomically to indicate classification, kind of is formal: Dingoes are a kind of wild dog.