There are two parts to being able to kneel after TKR's. First is getting back the ability to bend your knee to the needed arc, this depends a lot on putting in enough push through effort during early PT. Some people may not achieve this due to excessive scar tissue or other complicating conditions.
The second is understanding that you are kneeling on a piece of metal which pushes on the surrounding tissue when you put pressure on it, so yes -- it will feel very uncomfortable unless you use a thick soft padding under the knee. When gardening, I use a boat safety cushion with loops that I can drag behind me. When I kneel, I try to position my kneecaps over the edge of the cushion, so I am actually putting pressure below the knee cap along the leg, not on the knee itself.