What's your definition of "successful"? Mac App Store volume is quite low, especially with upfront paid apps. You're likely averaging only a few unit sales per day, right? Maybe only one unit per day, and even that might be a generous estimate.
> A few open-source clones have also appeared on GitHub
Is your app open source? If so, that's probably why you're getting copycats.
> It seems that derivative apps with plagiarized descriptions and app elements are being approved without issue. Does this signal a shift in App curation?
No. Apple's so-called "curation" has always been terrible.