No, please don’t. Do it using word choice and tone of your writing. Sarcasm exists for a reason and denoting it with a single symbol is a bad idea. Sarcasm functions through its subtlety, in writer and reader.

Sarcasm is using categorical imprecision to point out how obvious the truth is. It’s words face-planting on purpose to get themselves out of the way of your eyes. To clearly label the sarcasm as such screws up this whole effect. It’s bird shit on the lens — now they’re looking at the surface of the lens not the thing you want to show them.

It’s a structure that points elsewhere, and requires an intuitive leap. Adding the /s bridges what should be a leap and the utility of the technique as a means of communication is lost.

If you find yourself tempted to use the /s, what you’re discussing is probably too important for sarcasm anyway and you should just say what needs to be said.