Comment by raisin_churn

Part of the huge cost to develop it was getting a mechanism that could deploy the mirror segments into a large enough array with sufficient accuracy, a task that would be dramatically simplified by a a launch platform with a wider fairing and greater mass to the L2 Lagrange point. So yes, if SpaceX Starship was available, it would've saved a ton of money on JWST, albeit mostly by relaxing the design parameters of the telescope rather than through cheaper launch costs.