Eye Diagram of QPSK Modulated Signal
Kindly suggest what changes I need to make to get a better resolution of eye diagram without increasing the sampling frequency?


There is none, since the resolution of your eye diagram is directly linked to the sampling rate of your observer. In your case, that observation rate is identical to the sampling rate of your signal-producing system.

However, in reality, you'd use an oscilloscope that has a sampling rate much higher than your transmitter.

You can simulate the same – just use a proper interpolator between your modulator's output and your eye diagram's input. Pay attention to the fact that an insufficiently designed interpolation filter will distort your signal – and that is exactly the reality of digital transmitters: the reconstruction filters after the DAC distort the signal, too, and so does the RF channel between a transmitter and the oscilloscope (and of course, the filters and active components inside an oscilloscope do that, too).