I'm curious if you want distortion or a filter sound? Or maybe some of both?
To be clear on terms, "distortion" is usually taken to mean a mathematically nonlinear function that creates harmonics that aren't naturally in the source material.
A filter can selectively make certain frequency components more or less audible (like filterbanks or audio EQ).
Modulated filters can create a form of distortion (like frequency shifting) but we don't often call it distortion since this term is often reserved primarily for waveshaping functions.
Maybe some more explanation for what type of effect you want to perform on the violin somebody can give some specific help. Perhaps the Biquad is the thing you want and you only need some ideas to make this change the sound of the violin in a way you can easily hear.
Maybe you want some distortion like suggested by @giuliomoro where you clip the signal and make it sound more like electric guitar.
For example, I have coded a variety of sound effects which can be implemented on Bela that somebody could use for many fun experiments with a violin. These aren't simple enough for me to recommend them as examples for learning how to use Bela. The examples provided by the Bela team are better for learning because they are short and focus on one thing at a time.
Everything works as-is, but you need to have something connected to the Analog Inputs to adjust levels or you might get settings stuck in very strange positions...and you need switches on digital inputs.