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.
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.