Why IR remote waveform is a saw-tooth or am I measuring it wrong
From the TI LM358 datasheet, the maximum output slew rate of your op-amp is about 0.3 V/us:



Your scope trace shows the signal has a slope of about 3 V per 10 us, pretty well matching the datasheet limit.

This is simply the fastest edge an LM358 is able to produce at its output.

To achieve a faster edge, you'll need to choose a different op-amp.