Yes there is a range of severity of symptoms. The severity of  the symptoms will vary depending upon the percentage of their cells that express a normal copy of the MECP2 gene.  If the active X chromosome that is carrying the defective gene is turned off in a large proportion of cells, the symptoms will be mild, but if a larger percentage of cells have the X chromosome with the normal MECP2 gene turned off, onset of the disorder may occur earlier and the symptoms may be more severe. Clinicians and researchers have also seen variances in the severity of the disorder depending on the type of mutation and where the mutation occurs on the X Chromosome. Sometimes this is referred to as the genotype/phenotype (the correlation between the gene mutation and the behaviour).  More information of the different mutations is available in this paper The association between behaviour and genotype in Rett Syndrome using the Australian Rett Syndrome Database.