No, Fuelly is focused solely on tracking fuel economy. We're not set up for tracking other costs.
If you just want to make a note of costs, you can always use the "Add a note" feature and put the cost of maintenance in the text of the note. But Fuelly wouldn't do anything with those numbers. The idea behind notes is to track things that affect your fuel economy.
Absolutely, service affects MPG. But the price of that service isn't as related to fuel economy. That's why we have the notes feature. You can use that to note when you've had oil changes, tire changes, etc.
Using the notes feature, describe the service and cost, and tag them with a "maintenance" or "service" tag. Then from "view all fill-ups" you can click on that tag and see all of the maintenance you've done and what it's cost you, as well as the odometer reading at the time of service.
I agree with PB that maintenance cost shouldn't be tracked by Fuelly. It would be used only sporadically among members, and then a "cost per mile" or total cost of ownership figure would be inaccurate when comparing one owner's car to the next. But, using the notes feature and tagging it you can keep very easy track of your total cost of ownership, including maintenance.