If Mr X earns better from a side job when compared to the income from his career option, does the side job still qualify to be called a side job?
Yes, it is still qualified to be called a side job especially when that jobs is not a contractual 8 hour a day job. A side job is a job where you have the control over your own time, whether you accumulate large amount of profit or not.
A side job is a great way to earn money especially if you have a lot of time after your fulltime job. Just make sure that this side job is a kind of job that is truly profitable and not a kind of side job that is a waste of time.