Well shoot, you can play the RIGHT scale over the RIGHT chord and still have it sound dissonant. It's as much a matter of what scale you use as it is a function of how you are phrasing the notes. Ever wonder why guitarists still scratch their heads at Django Reinhardt? It's because improvising is a seriously difficult thing to do--it's beyond the right scales.

A phrase in G mixolydian that starts on G, does some widdly-woos through the scale, and ends on A probably won't sound that great, since a major second over a major chord doesn't sound all that great. ((In terms of classical interpretations of consonance and dissonance o'course))

Don't forget you can just be lazy and alter the existing scales in some way or another. Just play C major aug 5 instead of C major if you're going for some dissonance. Hell, play Csus4 or Csus2 arpeggios over C major. You don't really need new scales, just alter the ones you know by one or two key degrees ((i.e. the ones that make them sound good))

The diminished scale, however, to my ear, is not as dissonant as some might think. It works over a staggering array of chords because of its symmetrical nature.

Trust me, with so many years of this instrument being played and millions of denizens---the possibilities for sounding bad are endless! :)