Yes, a side job can be very helpful if what we do doesn't interfere with our main job in terms of time, energy, and our head. For office staff, maybe they can still save some energy to do other work later. But if we talk about all the jobs (including heavy work such as carpentry, mechanics, construction), it will be very hard if you have worked all day and have to take care of other things.
This is one of the risks that we have to go through because regardless of the conditions, if we do main and side jobs then inevitably this has to be draining in terms of time, energy and mind so managing time properly is one of the right things.
But it can be paid off with the results that of course we achieve because it is certain that by working on the side we will earn more than the wages we get.
As long as we're not hampered by that and as long as we can share our time I think that's a good thing.
Currently I work in an office as a main job and have several sidelines in terms of Internet-based. I'm comfortable with it even though it's a bit of a sacrifice of time as long as I think I can do this then it's not a problem.