I agree with MorallyIncorrect: I don't really think Trubbish and Garbodor are the laziest designs in the Pokemon universe. They're bags of trash come to life. Simple concept, pretty simple design. Same with Voltorb and Electrode; they're Poke balls that somehow gained sentience. I think some of the laziest Pokemon designs I've ever seen are something that you'd figure would be more complex than what it actually is. Like Sudowoodo; this little thing with 3 green rocks on each finger mimics a tree... how exactly? Really look at it:
I'd assume something that mimics a tree would look a bit more like a tree. It's a stick figure with 3 green fingers on each hand. Another one is Luvdisc; it's literally a heart with a mouth and two eyes. You couldn't get any more simple. Aren't angel fish supposed to look a little less bland and simple? At least have some visible scales on the thing. And finally there's Klefki, which is officially Game Freak running out of ideas.
It's not so much the simplicity of the design that bothers me, it's the simplicity of the design matched with what the Pokemon is supposed to be. A tree mimic is supposed to look a bit like a tree. A fish isn't just supposed to look like a heart; the thing doesn't even have any fins. And then there's a Pokemon based on a bunch of keys. It can't be that hard to come up with something better than Klefki's design.