While those roots are correct, decalcomania is actually an 1860s borrowing from the French décalcomanie. You’d have to look at how -mania or -manie has been used in French to get an answer.
My speculation though, is that it’s just the “enthusiasm, craze” sense. Evidently it was something of a fad in the early 1860s.
Here’s an early cite, in English, of the French word that’s not in the OED, from Eleanor’s Victory, by Mary Elizabeth Braddon, 1863:
If a young gentleman now-a-days spends money in the purchase of a private box at Covent Garden, and an extra guinea for a bouquet, or procures tickets for a fashionable flower show, and is content to pass the better part of his mornings amidst the expensive litter of a drawing-room, watching his beloved in the messy mysteries of Decalcomanie, he may be supposed to be quite as sincerely devoted as if he were to plant his ladies point-lace parasol cover in his helmet, and gallop away with a view to having his head split open in her service.
