Not necessarily. I've witnessed people in an Apple Store choose a 5c over a 5s. I even overheard one person buying one say they preferred it. Not everyone cares about the latest, greatest.
If both phones were available at the same price, I'd wager there would still be a lot of people buying the 5c. It's not about WANTING plastic, maybe they just prefer the look of it?
As for those who buy one then throw a case on it, yeah I'd imagine a lot of them got the cheaper phone and are maybe hiding the fact it's a 5c (though I personally don't see why they should feel the need to do that). Others are probably using a case for the same reason most other people do - to protect it.
Don't get me wrong, I hated the 5C at first but I'm considering it now. I think it's a nice phone especially since I'm on a 4S.
However I want one because it's price and like I said earlier if it were 50-100 dollars cheaper i'd get one off contract. It would be more of a gap filler until the 6 and use the upgrade for that.
Personally I don't care if it's plastic or metal. I prefer features and internal upgrades that's my draw to the 5S.