US eugenics programs (which were usually state-based, not federal, with the exception of immigration restriction) were varied, and some of them targeted poor whites, but they targeted many other groups as well. I am not sure I would use "mostly" here; it depends on the program.

But in short: eugenics programs tended to target whichever group created anxiety amongst the class of people who had the wherewithal to advocate for eugenics programs. In the northeast in the 1910s the anxiety was directed at immigrants from south and eastern Europe, and the eugenics of that period (e.g. Madison Grant, Immigration Restriction Act of 1924) reflects this. During the Great Migration, concerns shifted towards African-Americans, and the eugenics literature changed to reflect that (e.g. Lothrop Stoddard). Amongst those whose interest in eugenics was crime and poverty (and conditions associated with each), the targets in many places were different. The primary people targeting poor whites were those of the Henry H. Goddard ilk. Goddard ran a state mental institution in New Jersey, and worried about the "feeble-minded" (people who seemed superficially "normal" but were really mentally deficient and harbored "deficient" genes). In the northeast, midwest, and the west, these tended to be the poor whites (esp. when you consider what Goddard would have considered "superficially normal" to mean in his day). (In the south, they were often African-Americans. In at least one state, Native Americans were targeted.)

Goddard's approach to eugenics was not the same as Grant's; he didn't buy into Grant's ideology about Nordicism (which would privilege the English as a group, though not as individuals), as far as I know, and even if he did, they were really talking about different issues (immigration vs. state institutionalization). Eugenics was a very broad umbrella in the early 20th century, and lots of social anxieties (of a variety of political positions) fell under it. It was not coherent, in the sense that very different groups could find some eugenic aspect to be interested in. Until the 1930s or so there was really very little distinction between "eugenics" and "people who thought thinking about human heredity was important." Only in the 1930s did the biological and genetics community start to separate out more distinctly and "eugenics" became synonymous with coercive reproductive restrictions that often targeted the vulnerable. (Even then, the term did not become positively toxic until the 1970s, even though it had fallen out of favor by the 1950s.)

All of which to is to say again: if you are looking for unity and coherency in American eugenics thinkers and programs, you will not find it. It was never "one thing." Even programs we like to link as a unitary entity, like the +64,000 compulsory sterilizations that were done in the name of eugenics, varied dramatically state by state, and in some states even institution by institution. In California, for example, not all of the sterilization operations were actually done in the name of eugenics — the California case exemplifies, in my mind, the lack of uniformity in American eugenics. It is in very stark contrast with a place like Nazi Germany which had a much more deliberately "coordinated" approach to eugenics.