Poor thing. Can you see that the skin is irritated under the mats or are you inferring based on the dog's reaction to being brushed. It may be that the dog is just scared of being brushed and yelps more from fear than from pain, though it's certainly understandable that trying to brush the mats would be painful. Clipping them may be more effective, since it doesn't require pulling at her fur.
Where you go from there depends on whether or not you feel the neglect is extensive or more accidental. You might mention in your updates "I noticed Mitzi has matting that seems to be causing her some pain. I trimmed/brushed them out and she's feeling much more comfortable!" to see how they react. They may just not know. There are still a lot of people out there who figure that dogs take care of themselves, not realizing that we've bred many of them to have coats which require brushing and cleaning periodically. Gently letting them know that her coat needs to be maintained for her comfort may be plenty.
If the problem is more severe, and the skin is irritated, infected, or worse, it requires more extensive action. Notify Rover, document with pictures, and again let the owners know what you see. Still be polite, but be clear about your findings and what needs to be done (ie vet care): "I noticed Fuzzbucket had some mats, and underneath her skin is irritated/infected/etc and very painful to her. This needs to be assessed by a vet as soon as possible. Please let me know how you wish to proceed, as it's very important she receive medical care for this issue." Conduct all your correspondence through Rover so it's documented.
If the owners are reluctant to seek care, again, notify Rover of your actions. If you decide to take her to the vet against their wishes, you'll want them to be aware. You can contact their vet to get their recommendation, and they may be able to contact the owners to urge them to agree. If the situation is even worse, and the owners adamantly refuse to get her treatment that you believe is necessary, you can take your documentation to your local animal control to have them follow up with the owners and enforce any applicable humane treatment laws.