If someone was creating music to fit a genre or to match a certain theme/mood would this influence their choice of which scale to use? Are some scales used more than others within a genre of music?
Are some scales used more than others within a genre of music?
Short answer: yes!
I think an exploration of the relationship between genre and tonality could probably fill several very large books - partly because the term 'genre' itself can cover many different axes. For example, we can look at genre from a geographic point of view, and see that many regions of the planet are associated with particular scales. We could also look at genre chronologically, and see how the development of western music ties in with the emergence of the equal-tempered 12-tone scale and the extended possibilities it allows.
We can also zoom in to particular genres and see that many of them are associated with particular scales. The archetypal blues sound comes from the use of a particular scale played by bending and inflecting notes played on certain scale degrees. Particular modes of the diatonic scale tend to be associated with European folk music.
We can't necessarily restrict our concept of tonality just to scale, though. Different genres create their own harmonic language through use of - or avoidance of - particular chords and cadences. Minor seventh chords feature heavily in funk and disco, for example. Rock musicians might avoid playing minor chords (even if playing in a minor key) because minor thirds can sound very harsh through distortion. The grunge music of the 80s and early 90s tended to avoid V-I cadences, and often featured particular chord voicings that were created by drop tuning. These are examples of features of harmony that can't be explained just by relating a genre to use of a particular scale.
If someone was creating music to fit a genre or to match a certain theme/mood would this influence their choice of which scale to use?
As discussed above, to write to a genre, you need to do more than just use a particular scale - you need to be aware of the chord vocabulary, chord voicings, and cadences (or avoidance thereof) that are typical for that genre, as well as how it might modulate or otherwise vary in tonality.
To some extent, you can point to scales that are commonly-understood to have certain "meanings" - such as the "minor=sad, major=happy" correlation, or the association of the whole-tone scale with dream sequences/flashbacks. But as well as thinking whether those assumptions are still valid within the genre you're working in, you also have to consider how your audience is going to perceive the statement you're making. Is your use of a particular scale really going to communicate the feeling you want, or is it just going to come across as a tedious cliché?
One of the problems with trying to associate any particular sonic feature with a particular function or emotion is that as soon as the association becomes strong enough for it to be commonly understood in a certain culture, there is artistic capital to be gained in working against that association in unexpected ways. This is one of the forces that has led to the creation of new genres of music.