Personally, I found the 5c colors nice. I dont see much of a difference to those huge (ugly?) cases that people wrap around their phones today. I dont know many people who are using naked phones today and my feeling is that phones today are not designed anymore to be case-less: the camera bump, razor-sharp edges, too thin bodies.
The 5c flopped because of other reasons: it was artificially made worse than the 5s. It started with 8GB memory that was already WAY TOO LITTLE back then, it had no touchid, it had a bad camera and so much more - and yet, the price was high. Apple learned from the mistakes and changed their segmentation strategy and it worked since then (with the exception of the weird XR thing).
I had an iPhone XR from work. It flopped…but it was a good phone, and it was not necessarily an obviously bad idea, in my opinion.
People like big phones. The idea of a big iPhone that has contemporary guts but lacks some of the really wild camera abilities of the Max models didn’t inherently seem like the wrong plan to me. But it just kinda went nowhere, probably because you can just go buy last year’s iPhone model.
It flopped because apple did that: "here is our standard model.. and by the way, here is a cheaper, crappier model".
Now they have the approach to have a full price standard model - and there is a super duper "pro" model for the extra successful, professional and rich people.
This approach makes more sense for the consumers and this makes the 12 and 13 fly the way they do.
The 5c flopped because of other reasons: it was artificially made worse than the 5s. It started with 8GB memory that was already WAY TOO LITTLE back then, it had no touchid, it had a bad camera and so much more - and yet, the price was high. Apple learned from the mistakes and changed their segmentation strategy and it worked since then (with the exception of the weird XR thing).