The output of the DAC is generally quite steady, except that there is a 'glitch' that occurs whenever it is updated.
Your code will be particularly noisy as you are updating the data to the DAC very frequently.
Here are some oscilloscope traces taken from an Uno R4 Minima running your code.
The blue trace is the input voltage on pin A5.
The yellow trace is the DAC output on pin A0.
Here is a constant 2.5V input on a faster time base, you can see each glitch:
There was a smilar topic at: hairy-output-on-r4-dac.