In-Universe, the Turtles reaction to the sea monster guarding the Kraang fortress falling in love with their submarine.
Depending on whether or not you believe the mutagen made the Turtles related to Splinter by blood, the fact that Karai is infact Splinter's daughter may have very well caused this reaction in some fans due to her relationship with Leo. Leo having a crush on her and Raph admitting at one point that she's hot "in the evil way" is this for some fans as a result. Not helping is the fact that Mikey openly refers to her as their sister after the reveal.
The fans who were guessing that Splinter and Karai are related since before the show even aired (due to promotional material) found the Leo+Karai gross from the start. Especially if they're in the party that believe the Turtles and Splinter are related thanks to the mutagen. The confirmation that yes, Karai is Splinter's daughter, caused several fans to abandon the Leo+Karai ship.
Some fans find any mutant+human Ship Tease to be a bit off-putting since most mutants, even if they were human-born, are very animalistic.
Concept art notes◊ on the Mousers reveal that their "gut" carries 5 Rat Carcasses. Ew.
"The Manhattan Project" reveals that mutagen comes from alien worms and the mutagen has to be milked from them.
At the end of "Target: April O' Neil" Mikey takes out a slice of pizza with bugs on it and eats it.
A Freeze-Frame Bonus in "Metalhead Rewired" reveals potato chips with flavors such as "Scab N' Sausage", "Menthol", "Salt N' Sweat", and "Mucous & Chives".
The pizza that Mikey bakes in "Pizza Face". Just look at it.◊ Understandably everyone was grossed out when Mikey actually ate it.
Don Vizioso's atrocious table manners, involving him spitting up an entire meatball that he was eating when speaking to Shredder.
In general, the mutations that happen throughout the series will either be this or Nightmare Fuel for the viewer.
◊ on the Mousers reveal that their "gut" carries 5 Rat Carcasses. Ew.
