Nothing new. A "comfortable middle class lifestyle" 300 years ago was probably that of a craftsman, farmer or petty merchant. Their comforts extended to a roof over their heads, usually enough food to eat, and probably being warm enough in winter. "Poverty" meant literally freezing or starving to death in a gutter.
Nowadays, "comfortable middle class lifestyle" means a 40 hour work week, and gets you a nice 4+ bedroom house with hot and cold running water, two indoor toilets, heating and air conditioning, 2+ motor vehicles, bigass television, internet, mobile phone, nice clothes, as much food as you can eat, and spare money and leisure to go on regular holidays. "Poverty" (for most of the world) is about on par with "middle class" 300 years ago. Working lots of hours, usually enough to eat, usually warm enough.
In a hundred years, "poverty" will probably mean "only has enough compute power allocated to simulate one virtual environment at a time", whereas "comfortable middle class" will be having your own actual cloud palace with stadium-sized holodeck.
They already do, to a large extent. I guess the OP should define what a comfortable middle-class lifestyle really is. If I had to hazard a guess, I would say: capacity to buy a decent house and car, pay off present and future debt and stay out of it, enough disposable income to meet life's necessities and yet have enough left over to save for a comparable lifestyle during retirement.
I don't think the UBI would provide all of that; instead it would be a means to avoid destitution (e.g. starvation, homelessness, unmet medical needs etc.) in the present. For all the luxuries that most of the middle-class hopes to achieve (luxury car, nice house, investments, vacations abroad etc) you would need more money than UBI.