What happens when an Imposter Mech, which entered the battlefield as a copy of a Theros God (e.g. Nylea, Keen-Eyed), is crewed? Will it be a creature, or does the phrase on the god which checks for devotion overrule the crew ability?
It will be a creature.
Those are two continuous effects: from the God's ability making it not a creature, and from the crew ability making it an artifact creature.
613.1. The values of an object’s characteristics are determined by starting with the actual object. For a card, that means the values of the characteristics printed on that card. For a token or a copy of a spell or card, that means the values of the characteristics defined by the effect that created it. Then all applicable continuous effects are applied in a series of layers in the following order:
613.1d Layer 4: Type-changing effects are applied. These include effects that change an object’s card type, subtype, and/or supertype.
Both of those continuous effects fall into layer 4, so we look at the next step:
613.3. Within layers 2–6, apply effects from characteristic-defining abilities first (see rule 604.3), then all other effects in timestamp order (see rule 613.7). Note that dependency may alter the order in which effects are applied within a layer. (See rule 613.8.)
Now we have to determine whether either effect is from a characteristic-defining ability.
604.3a A static ability is a characteristic-defining ability if it meets the following criteria: (1) It defines an object’s colors, subtypes, power, or toughness; (2) it is printed on the card it affects, it was granted to the token it affects by the effect that created the token, or it was acquired by the object it affects as the result of a copy effect or text-changing effect; (3) it does not directly affect the characteristics of any other objects; (4) it is not an ability that an object grants to itself; and (5) it does not set the values of such characteristics only if certain conditions are met.
No, neither effect is from a characteristic-defining ability: in particular, the God's ability does not satisfy (5).
Therefore, the two continuous effects are applied in timestamp order.
613.7a A continuous effect generated by a static ability has the same timestamp as the object the static ability is on, or the timestamp of the effect that created the ability, whichever is later. If the effect that created the ability has the later timestamp and the object the ability is on receives a new timestamp, each continuous effect generated by static abilities of that object receives a new timestamp as well, but the relative order of those timestamps remains the same.
The timestamp of the God's ability's effect is the same as the timestamp of the permanent in question...
613.7d An object receives a timestamp at the time it enters a zone.
...which is of the time it entered the battlefield (unless something unusual has happened to it that gives it a new timestamp).
613.7b A continuous effect generated by the resolution of a spell or ability receives a timestamp at the time it’s created.
The timestamp of the crew ability's effect is of the time it resolved, which is later. Therefore, this effect is applied last, and the permanent ends up being an artifact creature.
There's one more thing to check. Rule 613.3 also said "Note that dependency may alter the order in which effects are applied within a layer. (See rule 613.8.)"
613.8. Within a layer or sublayer, determining which order effects are applied in is sometimes done using a dependency system. If a dependency exists, it will override the timestamp system.
613.8a An effect is said to “depend on” another if (a) it’s applied in the same layer (and, if applicable, sublayer) as the other effect; (b) applying the other would change the text or the existence of the first effect, what it applies to, or what it does to any of the things it applies to; and (c) neither effect is from a characteristic-defining ability or both effects are from characteristic-defining abilities. Otherwise, the effect is considered to be independent of the other effect.
There is no dependency between these continuous effects (as (b) is not the case).