Does spell mastery affect summons?

Spell mastery affects your chance to "hit" any offensive spell, ie not get it resisted. The only time I can think of when it would boost the power of a summon spell is with the Touch of Entropy spell, which must hit and kill its target in order to summon a lurk.

Several tactical summons have a secondary effect on adjacent enemies, but this is not resist-able.

What Heavenfall said. There are various mage traits and some magic items which increase the level for summoned creaturesl; they're all clearly labelled as such.

The one thing which I would add is that spell mastery has no direct effect on damage either. It is compared to spell resistance to determines whether a spell is resisted or not. Generally spells which can be resisted like this (like Curse or Blindness) have no effect if they are resisted. The ultimate example is Kill, which costs hundreds of mana, takes several turns to cast, and if resisted will have zero effect. However if it succeeds it will kill any target, including elemental lords etc.

There are some other spells which have reduced damage (generally half) if resisted, e.g. Drain Life. Off the top of my head this only affects the Death spells though, the ones which do elemental damage (fire, cold, etc.) are not affected by spell mastery (although they will be reduced by any relevant fire or cold resistance).

This means Spell Mastery is most important for Death damage dealers, and fairly irrelevant for Fire or summoning focused mages. There are almost always a few spells which are worth having high spell mastery for, Titan's Breath for an air mage springs to mind, but it's much more relevant for some mage types than others.

Spell Mastery is very important if you plan to use Tame to get monsters on your side

One other thing, the main point of the Death spell Shadowbolt (especially since they nerfed the damage) is to to reduce spell resistance. So, for example, with a tough opponent with high spell resistance, it's a viable strategy with a Death mage to hit them with a couple of Shadowbolts before casting Curse, Blindness, Kill etc.