“Ambrosial” means “relating to the gods” in general, and does not only apply to their food.
ἀμβρόσιος […] poetic form of ἄμβροτος immortal, divine, rarely of persons […] in Homer, night and sleep are called ambrosial, divine, as gifts of the gods […] further, everything belonging to the gods is called ambrosial, as their hair, Il. 1. 529 their robes, sandals, 5. 338, 21. 507, 24. 341; their anointing oil, 14. 172, 23. 187; their voice and song, h. Hom. 27. 18, Hes. Th. 69; the fodder and the mangers of their horses, Il. 5. 369, 8. 434:—also, of all things divinely excellent or beautiful, Od. 18. 193
George Liddell & Robert Scott (1890). A Greek–English Lexicon, p. 74. Oxford: Clarendon.
The same meanings apply in English:
ambrosial, adj. 2.a. Of or relating to heaven or paradise; heavenly, celestial; divine.
b. spec. Chiefly Classical Mythology. Of food, drink, or anointing oil: of, relating to, or worthy of the gods; divine. Also occasionally of hair: anointed or perfumed with exquisite oil.
Oxford English Dictionary.
So Hera’s robe is “ambrosial” because Hera is a goddess and therefore everything relating to her is ambrosial in this sense. No doubt her earrings are ambrosial too, but Homer probably felt he had made his point.
As for the “ambrosia” that Hera washes herself with on line 170, Liddell and Scott write:
ἀμβροσία […] Ambrosia (i.e. immortality […]), the food of the gods, as nectar was their drink […] therefore withheld from mortals, as containing the principle of immortality Od. 5. 93 […] It was sometimes used as an unguent, Il. 14. 170 […] 2. in religious rites, a mixture of water, oil, and various fruits Ath. 473 C; and so some understand it in Il. 14. 170.
This is the passage from Athanaeus:
There is also the καδίσκος. Philemon, in his treatise before mentioned, says that this too is a species of cup. And it is a vessel in which they place the Ctesian Jupiters, as Anticlides says, in his Book on Omens, where he writes,— “The statuettes of Jupiter Ctesius ought to be erected in this manner. One ought to place a new cadiscus with two ears …—and crown the ears with white wool; and on the right shoulder, and on the forehead … and put on it what you find there, and pour ambrosia over it. But ambrosia is compounded of pure water, and oil, and all kinds of fruits; and these you must pour over.”
Athanaeus of Naucratis (c. 200). The Deipnosophists; or, Banquet of the Learned, volume II, p. 754. Translated by C. D. Yonge (1854). London: Henry G. Bohn.
Anticlides, from whom Athanaeus quotes this description of the composition of ambrosia, lived around the third century BCE, so there may be no connection with Hera’s bath in the Iliad, which had been set down four or five centuries previously.