I think Jason is intentionally meant to be stoic and "boring" because of Claude.
GTA 6s story is going to be similar to GTA 3s Bonnie and Clyde story. If you try to draw parallels between Jason and Lucia to Claude and Catalina, there's things that are different and the same. One thing I think that rockstar may have intentionally made similar in Jason is Claude's stoicism and quiet but piercing aggression and rage. I think GTA 6 will be a revenge story but rockstar has purposely gatekept the story from trailers as to not give it away. Jason's facial expressions always have been showing he's upset or having internal conflict inside, and his flat voice expresses that further.