Yes, it's good to wash the fabric before applying the acrylic paint. Washing the cloth does not only clean it but is also useful for the colour to stay for a long time without affecting the quality. However, some points are necessary to keep in mind. These are the following:
Make sure that the fabric is dried fully The material should not have shrinking or stretch effects; instead, it should be smooth and soft After drying up the material, ironing it can offer good results