Well, that depends on the amount the jobs pay, doesn't it? Have average salaries for manufacturing jobs (with respect to inflation) increased, decreased, or remained the same over the past 20 or so years?
That's how you can have an increase in the number of jobs while simultaneously collapsing a middle class. You can also convert full-time positions with benefits to part-time positions without, decrease sick and vacation days, require people lucky enough to have health benefits pay increasing amounts for them, etc., not to mention taking actions that simply raise stress in people's lives like making people work more erratic shifts, threatening them with off-shoring or outsourcing, basically any psychological gambit that makes the employee feel powerless - which has the follow-on effect of making them too cowed to asked for a fair share of the company's profits, again leading to less money for what was equivalent or better work. Plus that latter thing makes it less likely that workers would organize as a labor block or politically in their communities - a fine multiple win for the factory owners vs. their employees.
So yes, I can see several ways that a middle class can be hollowed out, even while increasing numbers of even worse, lower-paid jobs are created (and taken). That you don't see how this doesn't make things better for most demonstrates that either you are unaware of how the real world has been working for quite a while or you have some sort of odd ideological ax to grind.