Having left-wing funding and even a focus on refuting right-wing disinformation doesn't mean the information they provide is inhernetly inaccurate. They may well be objective enough to do their jobs honestly. Their work won't be balanced, i.e., they're unlikely to refute disinformation from the left, but that doesn't mean their facts are wrong when they expose right-wing lies.
That said, I agree that their funding and stated mission certainly mean Mediamatters reports should be considered with a grain of salt. If you want to understand the whole story, you should probably balance Mediamatters with other sources of good information. I'm just not sure where that is on the right.
It's certainly not Fox -- they have a long track record of presenting willfully false and highly distorted information. That's why they are so scorned by smart people. If you see a story on Fox, you always need to double-check it elsewhere to see how accurate it is ... or far too often is not. That makes Fox pretty useless in my book. The same goes for tabloids like Washington Times and Newsmax. They may sometimes print honest stories, but that's not a smart bet.
So finding solid sources is challenging. The mainstream corporate media are sometimes good, but their stories are usually fairly shallow unless it's Page 1 (and often even then). They also too often serve as a rubber-stamp for the status quo and focus more on sensationalism than substance. Getting more in-depth information for me usually means turning to Google and finding multiple sources. It also often means turning to more specialized sources, e.g., for specific industry publications, or to original source materials for studies, investigative reports, polls, etc.
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