Why were the tiles on the platform extensions on the IRT so drab and ugly? A product of the times?
When the IRT was built, especially the first phases, the stations were conceived as important public spaces, and they were designed as works of art.
Over time, for a variety of reasons, station design evolved into a "Just the facts, ma'am" approach. By the time many of the IRT platform extensions were done, the subway system overall was rather dilapidated, and no one cared whether the stations looked decent.
There was also a lack of appreciation for what the original designers achieved, and even when stations started getting full makeovers in the 1980s, the original designs were in many cases ignored (e.g., Bowling Green, Wall Street). Interestingly, there are now plans at the Wall Street (4/5) station to reverse the 1980s rehab, and restore the original design.
Anyhow, by the 1990s the MTA had figured out what Heins and Lafarge knew from the beginning - that subway stations are important public places. Most of the recent rehabs have once again treated the stations as works of architecture, not merely as utilitarian structures.