Yes—Patch 121 is cumulative. You do not need patches 116, 117, 118, or 120. However, you must have the base Platinum mod installed first.
Warning: Patch 121 is freeware. It should never be behind a paywall. Do not buy this from sketchy websites—it is a fan-made labor of love. test drive unlimited platinum patch update 121 free
Even with a perfect install, you might hit a snag. Here are fixes for the top three problems reported after Patch 121: Yes—Patch 121 is cumulative
Problem: “Game opens to black screen then crashes.” Problem: “My saved game says ‘Corrupted Data’ after
Problem: “My saved game says ‘Corrupted Data’ after patching.”
Problem: “Cars have no sound or engine noise is buzzing.”