We aren't really whizzes with a pastry bag. In fact, we don't own one. But we think we could get this look using a plastic bag with the corner snipped off. These dots don't have to be too precise, or even the same size, to be charming...Polka dots are just cute; there's no way around that. Dots of varying sizes all over a layer cake is simple enough to do, and they can hide imperfections in your icing (if, say, you have a few crumbs that got rustled up). These dots look relatively uniform, either a speck or a button-sized spot, and we'd strive for something similar to keep it from looking too random. But the point is that you don't have to be perfect. A few bigger blobs here and there won't look odd. And you can always add in tiny points to fill in empty space.
If you don't want to dye an entire batch of frosting, you could ice the cake in white, then do colored dots. Maybe pale yellow or pink? We would keep it to a single color, though. Multi-colored dots would be a different look altogether- less refined, more kid's birthday. Which isn't bad, of course, just different.
This has got to be easier than trying to pipe on flowers or leaves or, heaven forbid, letters.