I think Pokemon creation has been on the same level as it has in Gen 1. There are good ones, and there are ones that are WTF worthy. When Gen 5 I think they went back to their Gen 1 roots and went back with the simple designs. This is where I think people may confuse 'simple' with 'unoriginal' when comparing the two generations.
The first generation started it all, yes, but taking a step back from things and looking at Pokemon without confining them to their generations, the designs seem pretty steady with a level of originality and simplicity to me. With gen five, it seems they used simple designs with keeping in mind to go with a more modern flow, as they did with their plotline: anti-war. Before it was usually beat said bad criminal group, not something quite as thought provoking as the B/W plotline. Anyway, just as time goes on, Pokemon are in the Pokemon universe and that is an evolving place just like the real world is, it's in constant change so Pokemon would naturally change too.
The Garbodor line is really another version of the Muk line. Persian is just some fancy cat, whereas Stoutland is an alternate version of a dog. If you want another obvious link for Gen 5 taking a more Gen 1 track, I'll show you these two:
Pidgey and Pidove
Three freakin' letters difference between a three member evolution line of a common bird Pokemon.
Of course, like anyone, I question the existence of Vanillish and such related Pokemon. However I also question Voltorb. It's design was based off of something within the Pokemon universe. Then when it evolved it's coloured inversed and it's eyes changed... yeah woo that's exciting...
Horsea is a play on Seahorse, Bellsprout is a walking flower, Mankey is a corruption of monkey, Ratatta is someone saying 'rat' with a stutter, and so on.
Let's also not forget the idea behind the design, because that's a big part of it too. Someone mentioned Litwick so I'm going to use it in my example. Firstly, I think Litwick is a neat idea [and also cute but not because I found a shiny one on my Black playthrough, no not at all ><] because it's the idea of a possessed candle burning souls it captures. To me, that's a simple idea, with a cool concept.
I think people are forgetting that the real world has some pretty unbelieveable creture's that exist. I think people need to put more things into perspective about Pokemon designs here. Again, I'm not saying all of them are great, or that if you think a Pokemon design isn't great because you simple don't like it, that's fine. All I'm saying is that I don't think people are really looking at Pokemon designs as a whole with the Gen 5 vs Gen 1 argument.