Mer-Bucky is kind of a sassy little shit.
Also the merfolk language is very literal and blunt for the most part, and since a lot of it is based in body language for nuance there is not really room for lying or dishonesty in the same way as with English, and they are able to detect a lot more of the physical state of the other like heartrate and stuff so kinda like with horse therapy a horse can tell if your communication doesn't agree with your body- John is wary about Humans because to him this always feels like deception and dishonesty, because he's always been on the end of things where a human is trying to trick and catch them.
Bucky, though, learned about human language from Steve as kids- Steve wanted to be friends! Steve was in a lot of physical pain and distress often, but it was unrelated to Bucky. He learned about words not matching literally in the context of politeness and also as jokes. It's really hard to translate a sense of humor, the special thing about Bucky and Steve's relationship is how they grew up together, so human culture is actually a part of Bucky that is positive.
John recognizes this as a good thing, and he's smart, he's very intelligent among his species... but he's also fighting the reality of his Alpha hormones making his temper short sometimes, and with his trauma and experience with humans. But he knows the best chance at surviving involves befriending humans, that not all humans are the ones causing trouble- and Sam also points out how most of the humans causing the real trouble for merfolk are the ones causing real trouble for more humans. He gets it, a lot. Communication and intersectionality is important, and also really hard.
And Bucky is like Yes Yes... we need some fun sometimes though. I want to feel good can we feel good sometimes please :(
Yes, yes... you fussy baby...
Yaaayy! put it in my fuck-hole 😈
Bucky! Don't call it that D: