Can I look at my deck before playing the Zombie Mason
It is indeed a blind play, you cannot look at the top of your deck before choosing what to play with Necromancer (or however else you came to play the Zombie Mason). However, unless a pile has been depleted or you have 'unique' cards in your deck (Knights, Castles, cards from Black Market, etc.) you can always choose to replace the trashed card with an identical copy from the supply. It's not a good outcome of the play, but you don't loose a card in that case.

At the start of the game, the possibility to get rid of a Copper or change an Estate into a Silver or other 3-cost card is really good. If you trash an Action card, no problem, another potential card to play with Necromancer.

Later in the game you might be wanting to play either other Action cards trashed, or another Zombie. But if you are ahead in scoring, accidentally trashing a Province and replacing it with another isn't a bad outcome anyway.