[Morthoron]

Considering nearly every effort at isolationism in the books ended in disaster, or came to a terrible end: The Girdle of Melian, Nargothrond, Gondolin, and even the Shire with the takeover by Sharkey and his ruffians, it would seem Tolkien often offers cautionary tales on isolationism.

After all, Ulmo warned Turgon through Tuor:

"Love not too well the work of thy hands and the devices of thy heart; and remember that the true hope of the Noldor lieth in the West, and cometh from the Sea."

And speaking of the Valar, even their well-intentioned detainment of the Eldar in Valinor eventually resulted in unmitigated disaster.

As Pitchwife mentioned, I don't think Bombadil was an isolationist so much as a stationary manifestation -- in Latin a genius loci, the protective spirit of a place (or as Tolkien inferred in his letters, the embodiment of the vanishing Oxfordshire countryside). He has relations with outsiders who are worth relating to, such as Farmer Maggot and Gandalf, and aids Frodo and the Hobbits (saving them from both Old Man Willow and the Barrow Wights). Since isolationism and xenophobia usually go hand in hand, I don't think that defines Tom at all.