With a touch, you reduce the target's mental faculties. Your successful melee touch attack applies a 1d6 penalty to the target's Intelligence, Wisdom, and Charisma scores. This penalty can't reduce any of these scores below 1.
Touch of Idiocy doesn't actually do ability damage, it applies a penalty. Penalties are not multipled on a critical hit, only damage and ability damage/drain.
A spell that requires an attack roll can score a critical hit. A spell attack that requires no attack roll cannot score a critical hit. If a spell causes ability damage or drain (see Special Abilities), the damage or drain is doubled on a critical hit.
Requires an attack roll, check.Deals damage or ability damage drain, roll twice. Nope, and nothing for penalties.
If it did do ability damage, then everyone is still wrong. And by that I mean, you would roll 2d6 and increase by 50%, not 1d6 x1.5 x2. You roll extra dice (and apply static bonuses to the dice) before adding them all up for criticals, not roll one set and multiply. In practice though its typically, multiply out the static and then roll the dice.
Now, some spells specify what happens to their conditions/etc if they have different effects on critical hits. For example, Frigid Touch/Stricken Heart's Stagger goes to 1 minute instead of 1 round.