Quite acceptable and in some cases the only option. I had a 1950's semi the bathroom tiles were fixed directly onto a block work wall the old fashione d way with a strong mortar there was no way of removing them without damagi ng the wall, so I simply tiled over the top and nothing came off ever. The type of adhesive used is critical not all will adhere, mainly the ready mix ed air drying type. Go to a specialist tile shop they will be able to advis e you I did use a ready mixed type but cannot remember the make, the ones i n the DIY sheds are usually not good enough.
Richard