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 "mature."

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 "mature," 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.

"Matured cream" is definitely a thing; it's known variously as sour cream or creme fraiche. Traditionally, these were produced by taking some unpasteurized cream and waiting for a little while while naturally occurring bacterial cultures go at it. Commercially, they are now produced by pasteurizing the cream, inoculating it with specific bacterial strains, and holding it at a temperature which is beneficial for those strains.

Pasteurized cream will not reliably ferment like raw cream, because it lacks the proper cultures (either naturally occurring or introduced). What you end up will probably not be toxic (but it might be toxic), will probably not be very tasty, and will be different each time.