Truth is, neither Ford nor GM put enough effort into their subcompacts.
HFII's catchphrase was "minicars, mini-profits", and Iacocca's "thrifting" on the Pinto led to the gas tank debacle. He ran and hid from the Pinto the rest of his life, but that one was squarely on his shoulders. The bean counters not fixing the problem because "paying off the claims would be cheaper" came back to bite them in the butt later on.
The Vega engine was basically an experiment, with buyers being the test cases. The warping cylinder head and lightning-quick rust-out didn't help. It's a shame, as the car did have a nice ride and looked good.
The Gremlin was easily the best of the three; being a truncated Hornet, it was overbuilt. They should have built a four off of their great six cylinder engine, but with the Jeep buyout couldn't afford it until later. They should have done it sooner; the VW/Audi based four used in '77-'79 was expensive junk, and the GM Iron Duke used in the Spirit derivative wasn't anything to write home about.
Chrysler looked into doing something similar by chopping the back end off of the Duster, but they couldn't build it cheaper than a Duster and would have had to sell it for less. There was no appreciable fuel economy gain either. The Plymouth Cricket was a non-starter (too small engine) and bringing in the Colt only opened the door for Mitsubishi...