Whether it's a good thing, and how good, depends on what happens to the people who had those jobs.

Imagine that there are a thousand people doing a horrible job for $20k/year. Now a machine of negligible cost comes along that can do the same job as one of them for $1k/year. Then, oversimplifying in some obvious ways, we have the following:

Option 1: Machines replace humans, workers take the benefits. Each of these people switches from doing the job for $20k/year to doing nothing for $19k/year. They're better off, their employer is exactly the same as before. This is better for some people and not worse for anyone. Of course it will never happen.

Option 2: Machines replace humans, owners take the benefits. Each of these people switches from doing the job for $20k/year to doing nothing for $0k/year. Maybe they can find other jobs, maybe not. Their employer is just $19k/year per worker better off. This is better for some people (the owners of the business) but much worse for the employees, at least in the short term. It is quite likely to happen.

(In this case, what probably happens next -- at least if there is competition -- is that the company lowers its prices somewhat. So now the business owners win and their customers win. In the long run these lower prices may lead to new jobs.)

Option 3: Something in between. A union negotiates a special deal, or the government steps in in the hope of reducing unemployment and disaffection, or something. Exactly what happens will vary but it's probably better for workers than 2 and worse than 1, better for owners than 1 and worse than 2, and probably a non-negligible fraction of the benefits get eaten up by administrative costs.

In the long run, all of these are probably better than leaving things as they are. In the short run -- say, a few decades -- option 2 (which is the most likely of the three, I think) means a thousand people out of work, and quite a lot of them may be unable to find other jobs. This may well be a bigger loss of net utility than the business owners' gain in wealth.

If machines end up taking everyone's jobs, that could be glorious (if it leads to lives of comfortable leisure for all) or terrible (if it leads to lives of comfortable leisure for people who are already wealthy enough not to need to work, and starvation for everyone else).

So: yes, a lot of jobs are pretty terrible and an optimal world without those jobs is much better than an optimal world with them. But we don't have the option of either sort of optimal world, we only get worlds designed by Moloch, and the Moloch-world without those jobs may be even worse than the Moloch-world with them.