So what if I'm in Pokemon Diamond and I'm facing Dialga, and I send out a Smeargle who knows the move Sketch. Dialga uses Roar of Time, so can I use sketch on that move to have Roar of Time on Smeargle?
Yes — Sketch can be used on any move other than Chatter and Z-moves starting from Gen 4, so it will be successful on Roar of Time.
Source
Roar of Time is a valid move for Smeargle to Sketch. That doesn't necessarily make it a good move to have on Smeargle, but it is possible. Smeargle has abysmal attacking stats, so its typically better as a lead/hazard stacker -- though how much you care about that depends on your interest in competitive play.
