Which attempt at conquest, the 1889-1896 one that ended in the Battle of Adwa and subsequent treaties, or the 1935-1941 attempt that succeeded in occupying cities and certain areas (and arguably accelerating the demise of the League of Nations) but not in breaking armed resistance? The ideological justification in both cases was more nationalist than anything else, with Italy being late to the game of colonialism in general. Although the inherent racism of that system of colonial conquest was present, Italy didn't really think of their colonial adventures as altruistically as other powers; settler appropriation of land (on the Algerian model) and the derivation of economic gain were more important motivators all the way through. The issues of Christianity and antiquity explained the receptiveness and sympathy of European courts and publics to Ethiopian leadership PR (which benefited Menelik II before 1896, and gave Haile Selassie I the moral standing to successfully defy Britain's own claims to conquering-power status after May 1941). Another problem existed relative to Ethiopia in 1935; the loss at Adwa was really a significant factor in Mussolini's need to reverse it and begin building that new Rome. That the economy of Italy was in shambles, with the double albatross of Fascism and the Depression, made it even more imperative to reach outward.

In the later case, the claimed casus belli was a smokescreen, which involved skirmishing over the Ogaden region, where Italy had built forts around 1930 but which was within Ethiopian territory. Though the League held both sides blameless in 1935, Mussolini demanded reparations and apologies, while the French gave the Italians a free hand to respond as they wished and the British remained fairly quiet--they recognized this as an Italian 'sphere' even though Ethiopia was in the League of Nations. Mussolini therefore decided to attack rather than arbitrate or accept a multipartite solution per the Tripartite Treaty of 1906 between France, Italy, and the UK on how they would handle Ethiopian relations. The driving concerns were matters of Italian domestic politics and nationalist expansion (plus revanchism), not so much like the more typical colonial ideology. [edit: I should add here that as Ras Tafari Makonen and as emperor after 1930, Haile Selassie was a vocal and aggressive champion of modernization who traveled widely, as had been his father Ras Makonen and Menelik II himself. So railroads, central banking, telegraphy and wireless, and even air travel existed within Ethiopia. It would have been a hard sell as a primitive land, because virtually every European power (and even the US and a bit from Japan) had taken part in fostering this work together with the Ethiopians, including Italy; they had a constitution with a legislature, had formally outlawed slavery, and were in the League of Nations.]

I defer to Ray Jonas's 2012 The Battle of Adwa for the details on the earlier issues of the 1889 treaty and the run-up to that fight, but for the latter, Marcus's History of Ethiopia (or maybe 152-157 in Bahru Zewde's second edition of A History of Modern Ethiopia, 2001) will give the basic details. Milkias & Mataferia, eds., The Battle of Adwa (2005) is another one that explores the primacy of Italy's 'glorious mission' to appropriate such lands (King Umberto's words in the 1890s, not unlike those of Count Crispi; see p.39) which remained much the same under the Fascists.

I'll see if I have any of the pre-Adwa primary source material in depth--there were undoubtedly other subjective discourses going on in Europe about Ethiopia and Italy's colonies, even if they weren't quite as prominent in making political decisions. But as a venue for the civilizing mission or the white man's burden, Ethiopia was (as you note) not a likely candidate, and such claims weren't very convincing if Italy were to make them.