As someone who worked in Ipswich for a few years and went to watch them regularly, I have followed their plight over the years.
It's pretty safe to say their demise was well in place long before Lambert was involved - started around the Roy Keane days and went downhill from there. Basically, when Marcus Evans was the owner.
I have no idea how good Lambert would be now, but I wouldn't judge him on his record at Portman Road.