When a land becomes a creature, is it untapped?
The creature will still be tapped because nothing has untapped it.
Tapped vs untapped is one of the four categories of statuses that a permanent has.  Status is not affected by permanent type, and changing permanent types does not make the permanent a new object.  See the relevant rules:

110.1. A permanent is a card or token on the battlefield. A permanent remains on the battlefield indefinitely. A card or token becomes a permanent as it enters the battlefield and it stops being a permanent as it’s moved to another zone by an effect or rule.
110.6. A permanent’s status is its physical state. There are four status categories, each of which has two possible values: tapped/untapped, flipped/unflipped, face up/face down, and phased in/phased out. Each permanent always has one of these values for each of these categories.
110.6c A permanent retains its status until a spell, ability, or turn-based action changes it, even if that status is not relevant to it.