Thai Lion Air Mon, Feb 04, 2019 Tokyo Narita, Japan NRT - 11:00 am Don Mueang Intl, Thailand DMK - 04:15 pm Mon, Feb 04, 2019 (19hrs waiting time) Tue, Feb 05, 2019 Don Mueang Intl, Thailand DMK - 11:35 am Kathmandu, Nepal KTM - 02:00 pm Tue, Feb 05, 2019 (I am Nepalese)
You do need a visa as you are transiting for more than 12 hours:
Passengers transiting at an airport in the Kingdom with a view to catching a scheduled connecting flight within 12 hours of their arrival without the intention of clearing immigration procedure, do not require a visa. They must however be inspected by the competent authorities upon arrival and must stay only in designated areas of the airport until their departure. All passengers transiting at an airport in the Kingdom for more than 12 hours must obtain a transit visa prior to their arrival.
You can read details of the Thai Embassy in Kathmandu, Nepal here.