Mainstream tickling would be tickling that occurs in a mass-audience, non sexual setting.
If the tickling takes place in a non-sexual way but for a limited audience, then it can only loosely be described as mainstream. If it occurs in a film/programme/magazine etc. in a sexual setting, then it CAN be described as mainstream, provided it is made for the appreciation of the majority. But mostly, mainstream tickling occurs in a non-sexual context.