They were not isolationist, at all. They founded Liberia. They supported western expansion. They extended the Monroe Doctrine to Hawaii.
What they were, however, was very wary of the slavery issue destroying the union. The majority of Whigs (although it was not unanimous) opposed the Annexation of Texas, for example, due to the fact that they feared it would lead to war with Mexico while simultaneously causing the uneasy balance between the north and south in the US to cause the nation to fracture.
It is difficult to look back upon the politics of those days from the perspective of today, because foreign policy played very little role in elections. The crede that all politics are local may be true today, but it was absolutely true back then. Parties won, and lost, elections primarily on issues close to home. And parties often would push very different messages in different parts of the country, taking advantage of the fact that unlike today, what a party was saying in South Carolina might not be heard in New Hampshire. The Whigs in particular would take both sides on an issue- one side to appeal to the Southern states, and one side to appeal to the Northern states.