The drop rate are not "better" at higher difficulty or later Acts. They simply drop higher tier items (not necessarily better items), and the total drop rate does not increase, or at least not significant enough to notice.
“Wouldn't that mean you have to excessively farm Normal”
Yes. The drop rate for most recipe, as well as most blue/purple items, are Act-based. For example, if one item has highest drop rate at Epic Typhon, in most cases, it will also have very high drop rates at other Epic A3 bosses (Typhon belong to Act 3 although now it sits in Act 4); and reduced (imo around 1/3) drop rates at Epic A4 bosses. Other than Epic A3 and A4, the drop rate for that item would be extremely low. There are some exceptions, for example the Battlemage set seem to drop both in Epic A3-A4 and Legendary A3-A4.
So if you are looking for ragnarok recipe, the first priority are all Act5 bosses of corresponding difficulty, and the second priority are Act1 bosses of next difficulty.