Smeargle, is someone I honestly find underwhelming. Even with the ability to Baton Pass Shell Smash, Quiver Dance, Shift Gear, Coil, Sun Growth, Cotton Guard, Tail Glow, Dragon Dance, Cosmic Power, or whatever else, it is a statistical mess. It could have been better if Moody wasn't banned, or if it had Prankster, or Magic Guard, or even Overcoat. While it doesn't pass as many possibilities as Smeargle, the fact that Mew is no longer an uber makes Smeargle very forgettable. The only thing that puts Smeargle above Mew is Spore. Anything else is a complete waste of time.
However, there is a way for Smeargle to shine. Double battles. With teammates using moves Wide Guard, Follow Me, and/or Rage Powder, Smeargle can can do what it needs to do with relative safety. It's not exactly guaranteed, but it does help Smeargle cripple with Spore and boost afterward. Another way to get Smeargle to show its colors is to use it in the middle of a Baton Pass chain. Behind something like Venomoth or Volbeat, Smeargle will show up with boosted stats from the start with the opponent crippled by Sleep Powder or Thunder Wave while possibly being behind a Substitute.
All in all, Smeargle can only switch in on something that is crippled. Smeargle is not fast enough to cripple enemies with Spore on its own. Not only that, a large number of Prankster abuseres have access to Taunt, which completely shuts down Smeargle's purpose in life.
Here's the bottom line: if you do not use a Baton Pass chain team, Smeargle is nothing but garbage. Use Mew instead. Mew, at the very least, can fend for itself. Do not get drawn into the infinite movepool. Smeargle does not have the stats to back it up in any way. Period. Otherwise, use Smeargle in spin-off games. That way, Smeargle can really be amazing.