The laughter of the Romans and their Chufles was certainly enough to make an ordinary man stop dying themselves blue, but the Picts were not ordinary men.
The real reason that the practice stopped was on account of the fact that polecats seemed to be particularly attracted to the scent of the woad. The polecats would become overpowered by the halucinogenic properties and end up in the woad vat. Now the Picts were not cowardly men and their women liked their men to have a strong musk, but not that strong. Therefore the pratice was given up. It incidentally inspired one of the all time great songs still heard today:
"There's A Dead Skunk In The Middle Of The Woad"