Mature for what use?
For what customer base?
If you mean academic uses, ebooks haven't even begun to address actual needs either in the K-12 or Higher Ed industry.
But if you mean recreational, narrative reading, then I'd suggest that not only are the formats in use mature enough, at least one is way past mature and into gold-plating territory.
Right now the overwhelming majority of ebook revenues come from plain-text narrative fiction, with a much smalller but still substantial business coming from consumer-grade non-fiction. For those markets Mobi seven is adequate, epub and KF8 are more than good enough, and epub3 would be overkill.
Other markets have the old chicken-n-egg quandary; sales aren't there so the content isn't made available and without content the sales don't improve. And without either the suitability/maturity of epub3 won't even get tested in the wild. so its a moot point.
A better point to ponder is what Harry alluded to: are specific publishers' workflows and business practices, especially quality-control, mature enough to consistently deliver a quality product? Some yes. Most, probably not.