For me, it is pretty much a hobby...
The answer to the hobby question depends on where a person is in their crossdressing. For each of us, CDing is on a continuum... some dabble a little and others are consumed 24/7.
There are probably points at which the "hobby" becomes more than that... just as a casual photographer over time may well become a professional. But that doesn't mean that the professional photog is just a serious hobbyist.
So if you are dressed 100% of the time, you have probably progressed beyond the hobby stage. But if you are content to wear panties or pantyhose once in a while... maybe sleep in a nightgown on occasion and be perfectly content and satisfied with that level of dressing... then it may well be a hobby for you as well.
I have little desire to go beyond my current level of dressing and would/could probably cut back on that if circumstances changed. That is obviously not the same for everyone here so it is hard to identify CDing as a hobby for everyone... but it probably is for some of us.
Lacy PJs