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In the first treatise of On the Genealogy of Morality, §5, Nietzsche proposes the following derivation of bonus (good):

I believe I may interpret the Latin bonus as "the warrior": assuing that I am correct in tracing bonus back to an older duonus (compare bellum = duellum = duen-lum, in which that duonus seems to me to be preserved). Bonus accordingly as man of strife, of division (duo), as man of war--one sees what it was about a man that constituted his "goodness" in ancient Rome. (translation by Clark and Swensen)

Obviously, a lot of important etymological scholarship by more level heads has happened since 1887. Is Nietzsche's proposed etymology for bonus plausible?

This etymology is not accepted by modern scholars, though the ultimate origin of "bonus" is contested. I cannot get de Vaan to scan properly, but here is Walde:

Here is a transcription from the German source in the image:

bonus „gut“, altlat. duonus, noch älter Duenos (nicht damit ablautend; vgl. Thurneysen KZ. XXXV, 204, Pedersen KZ. XXXVI, 91), vgl. auch bene, bellus: nach Darmesteter De conjug. latini verbi dare (1876), 26 ff. , Osthoff MU. IV, 370 ptc. auf -eno- zur Wurzel von ai. dúvaḥ n. „Gabe, Ehrerweisung“, duvasyáti „ehrt, verehrt, erkennt an, belohnt“, duvasyú-ḥ, duvōyú-ḥ „verehrend, ehrerbietig“, wozu auch lat. beāre „beglücken, erquicken“, beātus „selig, glücklich“ ; hierher nach Wood Mod. Philology IV, 499 als idg. *du̯ei-to- vermutlich auch mnd. twīden „willfahren, gewähren, bewilligen, erhören“, mhd. zwīden ds., md. getwedic „zahm, willfährig“, vgl. auch as. tu(g)iđon „gewähren“, ags. tygđian, tīđian ds.; die Bed. „durch Gaben, durch Beschenkung ehren oder erfreuen“ (woraus die Bed. von ital. *du̯-ei̯ō, *du̯-enos sich leicht ergibt) ist der Hirt'schen (IF. XXI, 169ff.) Anknüpfung der Sippe an alat. duim usw. „ich möge geben“ (s. dō; er führt idg. *dō-, *dōu̯- auf älteres *dou̯e- zurück) günstig. Sehr fraglich ist Zugehörigkeit von dautia, lautia, s. d.

Wenig ansprechend ist Fröhdes BB. IX, 111, Ficks I⁴, 457, II⁴, 150 (ebenso Prellwitz Gr. Wb. unter δύναμαι) Verbindung von bonus mit einer Wz. *dū-:*deu̯ā- „stark, fest“ in gr. δύναμαι.

To answer the question; of course it is.
You don't have to be of the same opinion of him, but the linguistic reasoning is sound. He has presented the logic very reasonably and here is a truth table:
if bellum = duellum then bonus = duonus.

Plus the ancients were fond of war: just consider the phrase "to have had a good war."

I agree with Octavius Octavian

@brianpck The logic is sound: du = du, and duellum - du = ellum, much in the same way that du = du, and duonus - du = onus. Now in both cases where b = d, the results are the same: bonus and bellum.

Let's not ignore the consistency of the 'goodness of war' being implied in the etymology of both words.