Being called foxy, in and of itself, isn’t a good thing or a bad thing, it actually depends on what time period the word was being used. For example, “fox” originally referred to a person’s cleverness, as in “sly like a fox”; then it came to mean not to be trusted, as a “fox in the hen house”. Now, being called foxy refers to a good looking woman, as referenced in Jimi Hendrix’ immortal song, “Foxy Lady”.
The process of a word or condition going from bad to worse is called pejoration.
In linguistics, pejoration refers to the process by which the meaning of a word becomes negative or less elevated over a period of time, for example “silly”, which formerly meant “deserving sympathy, helpless or simple,” which has come to mean “showing a lack of good sense, frivolous.”
Etymology: From Medieval Latin (pirti); from Late Latin (pirtus), past participle of pirre, to make worse; from Latin pior, worse.
Source: The American Heritage Dictionary