pic16f877a problem in timers with proteus simulation
The code posted sets TIMER1 to be clocked from the FOSC/4 source and to assert an interrupt after 65536 clocks. 

The interrupt handler toggles the state of RB0. 

With a 16MHz oscillator the TIMER1 input clock is 4MHz, Divide 4MHz by 65536 the interrupt asserts every 16.384 milliseconds.

The signal on RB0 does a full cycle every 32.768 milliseconds or a frequency of 30.51Hz. 

The results of your Proteus simulation seems correct.