Unfortunately the Travelcard is not commonly accepted in Nepal as most merchants do not accept MasterCard (even if they say so). Visa is commonly accepted though, so I propose you bring both your Travelcard and a Visa card as a backup.
In Kathmandu it's relatively common to pay by card, but remember that the average amount for food, drinks and other things will be relatively low. As a result, most merchants will ask you if you can pay by cash.
If you want to go to an ATM, keep in mind that most charge a 500 Nepali rupee fee (~4 EUR) per transaction for foreign cards, including the Travelcard, on top of regular transaction fees.
