It's a monostable circuit , the output of the monostable when triggered it what the O-scope pictures are, but what is causing the monostable waveform to be distorted?
Your response lacks sufficient detail to enable suggestions about which element(s) of your design or test set-up are causing the distortions. There are several ways to produce a monostable pulse. Here are some: https://www.electronics-tutorials.ws/waveforms/monostable.html
Can you show your actual circuit, including how you have the oscilloscope attached?
In case we are talking about the classic monostable made from discrete components...
The action is not entirely on-and-off. If it were, then the output pulse would be 'clean'.
Instead the action is governed by a capacitor which charges and discharges. It controls the bias to a transistor.
There is a point in the cycle where the bias is barely sufficient to keep the transistor turned on. That is where the output pulse starts to appear 'messy'.