Hi Philipp,
I asked my brother, he is the one the worked on the animation, here is his answer:
Hey Philipp!
Thanks for your wisheswe are so glad you like our game!
Honestly I think that a correct balance between sprite swapping, mesh deformations and bone animations can be found accordingly to the style you want to get in the end.
My habit using Spine is to focus on animating bones mostly. Then I start tweaking the curves to get some nice accelerations and to exaggerate almost every movement a little bit, so they look more natural and catchy. Then, usually, if some more perspective is needed, I add some mesh deformation to give it a final retouch. I always keep the sprite swapping as a “last chance”. It’s of course needed when, for instance, you have a drastic change of perspective, like the case of the cat you are about to animate.
What I suppose you are concerned about is how to properly blend a texture swapping (that may look like a stutter) into a smooth animation, right? In those cases what I usually do is to make that swap happen in the middle of a really fast movement, so that the discontinuity becomes less noticeable.
To animate the main character of our game, for instance, I had to swap textures for the feet (during the jumping animation) and for the mouth (to match the various voices). What I did was just to locate the swap in the middle of some fast movements. To animate the mouth, for example, I used to aid the swap with some scale. Let’s say I have to pass from a wide open smiling mouth to an almost closed “o-shaped” one. What I would do is to scale down the smile so that it almost matches the size of the second one, than make the swap. Eventually, the “o-shaped“ mouth could be a little bit up-scaled when the swap occurs, so that the transition looks even more smooth. All this process would be happening in something about 5 to 10 frames, so it’s really fast and the texture swap becomes almost invisible.
Animating the octopus I actually did the same. When I started animating it, I was not so comfortable using meshes, so I preferred using the process I just described to you. Right now I would strongly suggest to take advantage of meshes to animate something like those tentacles. What I did was to put the swap in the middle of some fast movement to make it less noticeable. In this case I’ve also put a motion blurred tentacle texture in the middle. It’s just barely noticeable when you play the animation, and of course it’s another piece of texture that has to be added to the final sprite sheet, but it helps keeping the result pretty smooth.
To animate the flowers I just to split the stem in multiple parts, making them scale up one after the other so that it looks like it’s growing up. I am pretty sure that using meshes would be helpful in this case as well, but even some scale can just do the job if does not create weird effects. In this case the texture was pretty simple, so there was no risk to get unexpected artifacts.
About the change of perspective of the cat you just mentioned, I think our carnivorous plant animation could be interesting for you. In this case I had to make this plant look in front, and then tilt the head back while opening the mouth. What I did was just to scale down eyes and forehead, slightly translate them up and fade away while the mouth is scaling up. As always I try to keep this “difficult” transition as fast as possible, so eventual imperfections are not noticeable.Hope my experience can be helpful J thanks again for appreciating our job!
I hope we could help you 
Keep it up!
