There's no doubt in my mind that Smeargle is by far the better card and there's no real question about it.
Cleffa's uses are barely even comparable to Smeargle's. Cleffa is normally used early game to help refresh your hand after playing down your first supporter, attaching your energy, and whatnot. It can also be used mid to late game if your hand is completely dead and you need something new to carry you through the rest of the game. An effective use yes, but using your attack per turn to refresh your hand instead of using said attack for a KO/setting up a KO can give your opponent an opportunity to get ahead in the prize trade or simply pull ahead even further on their following turn. Not to mention Darkrai EX has become popular so if you have a Cleffa sitting on the bench it's an easy prize card for your opponent
Smeargle on the other hand has a lot more going for it. Instead of using your attack for the turn, Smeargle can just sit the bench and be used to help you out every turn whenever needed. The one retreat cost doesn't even hurt it with most of the top decks playing Skyarrow Bridge and Switch now. One of the simplest ways to use Portrait is to promote Smeargle after one of your active Pokemon gets KO'd. With 2 Smeargle, Skyarrow Bridge, and Switch in your deck, abusing Portrait becomes extremely easy to and gives you a potential 3 supporters in 1 turn which is just incredible. The only small downsides I can see is not having Skyarrow Bridge or Switch at the correct time and of course the fact that your opponent may not have a supporter, but I find that most of the time my opponent usually has some form of supporter sitting in their hand for their next turn. In the event that they don't have a supporter, you still get to take a look at what's in your opponent's hand which is quite useful for many other reasons.
The ability to use more than one supporter in a single turn is almost too good to pass up. In this fast paced format where setting up quickly should normally be your number one priority, being able to use an extra supporter early on can make all the difference when it comes to setting up and taking your first prize before your opponent.