Although Penguin and Riddler have a "best friend relationship" in the show, in the comics it was actually Penguin and the Joker that shared this bond.
The telescope that Tetch gets Gordon to look through is inscribed with the phrase "Through the Looking Glass," a blatant addition to the obvious tie-ins between Tetch's Mad Hatter and the tales of Lewis Carroll's "Alice in Wonderland"
Kieran Mulcare, who plays the White Rabbit in this episode, is one of the rare Marvel-to-DC crossovers. Previously, he played the smitten upstairs neighbor to Jessica Jones (2015) on Netflix.