60 fps is currently the standard in the animation tools that Playtonic uses (Autodesk Maya, for example). It may be that the Unity Engine is throttling the framerate of Yooka and Laylee's animations to 60 fps, as to prevent animation errors as you're spinning the camera about. As you spin the camera, you are rapidly introducing objects the engine must render, and to prevent a slowdown throughout the whole game, the engine could be cutting corners on the player model, since it figures you'll be paying attention to the spinning surroundings, rather than Yooka and Laylee. Don't take my word on it though.
On that note, this isn't a massive issue, in the grand scheme of things. Is it truly vital for a stylized game like Yooka-Laylee to run at 144 fps at all times? I imagine Playtonic would simply cap the game at 60 fps if the frame drop was that much of a problem.