It's not as black and white as saying Button is a liar or not. That's a very strong word and ignores the reality that life has shades of grey. The teams put a positive spin on everything, its how they earn their sponsorship funds.
As you said, its a mixture of many "sucky things". Button saying he wants a car that is more/less under/oversteery in the winter is probably only a small "sucky thing" compared to the fundamental errors in the design of this car. Perhaps his feedback during the season isn't the best, but the guilty party when it comes to a fundamental lack of downforce must surely be the design team?
Note this is the same team that a few years ago went from best car on the grid to dead duck the following season. It is remarkable that a team could fall on their face twice in a matter of 5 years. Drivers come and go, but the same design team made the same mistake.