"Little" in the context of kink means the opposite of mommy/daddy. It's a roleplay thing. Personally, I can easily imagine Kiwi wanting to be pampered or patronized in that way.
The "meme" is just that there have been two anime in very recent memory that both have elf spellcasters who have a human spellcaster companion who is stacked. It's a "two nickels" kind of thing
I'm currently reading this series again, and yes, Tomo is leagues stupider than Osaka could ever be. I'm pretty sure Osaka is smarter than Kagura, too.