I wouldn't say a person was either of those words. But I'd say that of someone's personality. I found her personality sickly-sweet. Her personality was cloying.

Hi Pablo, These adjectives are more naturally used to describe behavior, especially speech. "Her words were cloying." That sort of thing.

Thank you so much, Rowena

Definitely coying. 'Sickly sweet' isn't incorrect, but I would usually expect it more when describing a flavour or other experience. (Incidentally, you probably know this, but the two phrases don't quite mean the same thing)