I am not aware that Hergé commented on this scene specifically, but its incongruity has been widely noted. In Tintin: The Complete Companion, Michael Farr notes that:
as The Broken Ear nears its conclusion, Hergé allows his imagination
to run away in a truly medieval manner. The villains of the adventure,
the bungling Ramon Bada and Alonso Perez sink to the bottom of the sea
and in the most fanciful image in all Tintin - except for dreams -
Hergé portrays three winged, horned and cloven-footed demons, armed
with two-pronged forks, dragging the unfortunate pair down to hell.
Such treatment is more typical of a medieval illuminated manuscript
than a twentieth century strip cartoon, and as such remains an anomaly
in the Tintin adventures.
Farr hypothesises that this scene was included to appeal to the young readers of the comic strip. At the time (long before the strips were published in albums as comic books in their own right) Tintin was serialised in Le Petit Vingtième, a conservative, Catholic newspaper distributed in Belgium. Farr writes that:
Devils and hell, however, would have seemed to the young Roman
Catholic readers of Le Petit Vingtième to be just and expected
retribution for evil
The two characters were responsible for an unusually high level of crime (at least two, and possibly three murders), which possibly accounts for their special fate.
Concerning the second part of the question, "Was this imagery ever objected to?", this did happen at least once. Tintin was syndicated to a French newspaper aimed at Catholic children, called Coeurs Vaillants. In Tintin and the World of Hergé, Benoît Peeters relates how the manager of the newspaper, Abbot Courtois, visited Hergé in Brussels to complain about this strip.
[Hergé's] relations with Abbot Courtois were never simple. He often
felt that Hergé's stories did not place enough emphasis on "Divine
Providence"; in some cases he went so far as to edit dialogue himself.
At the end of The Broken Ear, when the two bandits die, he actually
asked Hergé to redraw the scene, and we see Tintin add with a faraway
look on his face, "God have mercy on them."
(graphics taken from the webpage https://tintinomania.com/tintin-cv-oreille-cassee-dieu-ame )
Hergé naturally resented this kind of meddling, commenting "On the surface it cost me nothing, but that kind of addition was really difficult for me."