Its not that solely responsible. While they had a large impact on the countries, the disfunction of many Central and Southern American countries is far deeper ingrained than a company that comes around in the late 1800's. The foundation of Spanish Speaking American underdevelopment comes from the stark divide between people of European and native descent in many of the countries. This is especially powerful in Central America, Peru and Bolivia. Even Mexico was dominated by this struggle until the rise of the PRI.

While United Fruit came in, it didn't have that much of an impact on the countries because the economy was already structured in a way that fit the UF model. Families of European descent monopolized political power and revenue from natural resources. All United Fruit did was increase the latter. While UF then did help stymie political change, IMHO it wouldn't have made that great of a difference. The foundations of political instability (ethnic tensions) would have still been there and barring a complete nationalization of the economy to wipe out the dominance of European based families, the economic problems would remain too. Even Mexico, which is a bit more advanced on ethnic matters than many Central American countries still struggles mightily with native poverty and political under-representation (see Chiapas political tension).

In short, Central and Southern American underdevelopment and poverty have far deeper roots than the United Fruit company and thanks to the pre existing model the Fruit company didn't even change things that much, they just slotted in perfectly to what was already happening.