The concept of “intonation” in an instrument really only applies if it generates fixed pitches (because it has frets, keys, valves, etc.). Then the concept could indicate how well the instrument stays in tune up the fingerboard (i.e. guitar or mandolin) or throughout the full range of notes (i.e. clarinet or trumpet). A poorly designed or damaged instrument can be in-tune with itself in one range or key and out-of-tune in another.
With an instrument like a violin or viola, that doesn’t really apply. Tune the strings, and then the intonation is up to the player.
Can the player can influence the “intonation” of instruments with frets or keys or valves? With many of these instruments, yes–but they do most of the work for you. On a bowed instrument, you have to earn every note. As a 30-year guitar player who recently picked up viola, this reality is a daily challenge!