Would this be called matured cream?
Most people would say it's gone bad, or gone off, yes. Since you didn't ferment it intentionally, there's no telling what it is now, and you should throw it out.
Cheese is a fermented food which develops slowly as it ages. Maturation isn't random or by chance. The environment is carefully controlled to ensure the fermentation process continues smoothly, and that continuation is desired. Since you didn't intentionally begin or control this fermentation process, I don't think anyone would consider it &quot;mature.&quot;
As far as other fermented dairy products go, they generally aren't intended to last as long. I suppose you could call an overripe creme fraiche &quot;mature,&quot; but probably wouldn't, simply due to the fact that maturity isn't desirable in a creme fraiche or buttermilk. Those products are fermented quickly and then stored in a way that attempts to halt the fermentation process.
So, no, you don't have matured cream because what you have isn't the result of intentional, controlled, and desired fermentation.