No you don't, all that's needed is the pins simply wiring together - in fact assuming it's a European set with SCART you don't even need to do that, it's already done inside the mono TV set.
I think it depends on what the OP wants to feed the mono TV with.
I some cases like a cell phone its bad to short the left and right audio channels together.
So to error on the side of caution, I suggested a resister based mixer circuit.
We will not realy know what the OP needs unless we get more info.