In my opinion, it depends on the objectives that want to be reached. There are many applications of torture: punishment, information prodding, humiliation, etc...
Broadly speaking, I'd say it wouldn't be very efficient for a number of reasons. Firstly, the varying levels of sensitivity between potential victims: a failed attempt at tickling a non-ticklish person would result in the opposite results the torturers are trying to achieve.
The second is more pragmatic: there are WAY more efficient methods of inflicting pain, distress and discomfort to a person than tickling. Waterboarding has been brought up: it does not require much in terms of logistics and is ruthlessly efficient. Electricity too. Beating the soles of the feet as well. All three cause an EXTREME level of distress, pain and humiliation in a short time, efficiently, and with minimum to no damage for the subject, especially if the torturer has been properly trained and a doctor is present to monitor the session.
This is essentially why tickling has not seen much use around the world as an efficient method.
HOWEVER, I can see two conditions under which tickling could be used as an efficient and reliable method to be used efficiently. If by the use of some kind of drug or enhancement a person could be made ultra-sensitive AND the tickling could be delivered in a reliably efficient manner (possibly through the use of a machine) it COULD possibly become more efficient than beating, waterboarding or electrocuting as it would cause a matching amount of distress and humiliation within the required time frame while preserving the subject's bodily integrity. There is no doubt that tickling is hell to a ticklish person and can sometimes be used as a form of bullying or abuse. But to take it to the next level, it would necessitate at the very least a cheap and efficient method to overcome its shortcomings.
Under the current state of affairs, I can see it at most combined with sensory deprivation as a method to augment the subject's feeling of being dehumanized. When blindfolded and possibly restrained, being prodded, manhandled and tickled from all sides might generate confusion, humiliation and the subject's feeling of being dispossessed of his/her body and agency. It could be a good preparation for worse things to come or before an interrogation, especially if the subject is ticklish. But not more than that at present, until technology steps up the tickling game. When it does, I hope someone will say: "who's laughing now?"
