It’s a word that gets used and has been used by most of the population for centuries and everybody knows what it mean. It doesn’t fit in formal English but most contractions don’t. It’s fine as informal writing. Really only English teachers care, and nobody really cares what they think.
If you write it in a business letter or on a job application the people who read it will care. It’s a notch about using “innit” for ‘isn’t it’.