Laziness can be caused by mental illness but laziness itself is not a mental illness.
Some people who are perfectly healthy and have no mental health issues or even depression are lazy and just do not want to do any work etc.
But depression and other mental disorders can be a cause of laziness or lack of motivation for anything.
Laziness and lack of motivation to do anything can be caused by a number of different things.
For example if you want to make money say blogging and you're not making any money right now then the cause of the lack of motivation to continue blogging would probably be because you're not earning money.
Or another example is you're working a job that you have lost interest in or have no interest in doing the tasks even though you're getting paid you still lack motivation.
I used to work a cashier job and I got really bored of it and was not getting motivated to go to work so I found another job and quit that job.
Some people are just lazy though and don't want to work or do anything at all and it's usually just because of the way a persons brain is wired.
Lack of motivation and laziness can be caused by a mental disorder as well and the person may be so depressed and dealing with mood swings etc that they cannot get the motivation or are just bored of what they were doing.