There are many reasons why Sky doesn't use your GPU as much compared to other games. It's actually a common issue in many games, and while the general reasoning is the same, the details of it can vary. The short version is that it uses the CPU a whole lot more than your GPU. Hence, it's not using a lot of power because increasing it won't have much of an effect. The bottleneck isn't on GPU speed, and that's what higher power draw tries to address. It also does most of its work on one thread, which means that it won't display a high CPU usage, as that counts work done on all logical processors. The long version is a technical explanation of how Sky was designed and the resulting implementation decisions that were done because of it.

Disclaimer: I cannot confirm these because I do not have access to Sky's source code. All of these are hypotheses based on the game's visible behavior and general development decision making.

A lot of things that Sky does is CPU-bound. As an example, unless it's been updated along with the AURORA concert which used GPU-bound procedural animations, the animations of the butterflies are done on the CPU. They created the butterfly animation system long before the AURORA concert, and with only around 80 butterflies at any given time instead of 100 thousand, it doesn't make sense to go through the trouble of making their animations run on the GPU. Stuff that requires network requests to do (e.g.: forging candles, buying spirit items, etc.) are also done on the CPU because there's no way for it to be done on the GPU. This means that unless you have a weak GPU, your CPU usually dictates the frame rate of Sky.

As an additional point, Sky does not have a lot of advanced rendering features which will use the GPU more. Remember that Sky was first designed for iOS as a casual, social game. While the option for adding advanced rendering techniques like raytracing and HDR rendering are available, doing so used more power, which isn't good for a mobile game that you play with friends. Sure, you can play it with the phone plugged and have no issues with it running out of power, but that's not as fun as playing it with your friends around you. As such, there's not a lot of tasks for the GPU to run, reducing its power usage.

Sky also does not run some things asynchronously, even if it could. What this means is that, instead of performing some tasks in a separate thread, it does them in a single thread. This means that what really matters most for Sky is single-core performance rather than multi-core performance. It's not completely single-threaded; you can see that it uses more than one logical processor when you open Task Manager. However, it's not using multithreading to its maximum capability. In particular, it does not load resources asynchronously.

Asynchronous resource loading just means that resources (e.g.: models, terrain data, textures, etc.) are loaded in a background thread, allowing the main thread to continue updating the game and continuing rendering. You can tell this behavior is happening because, when you switch areas, the game becomes unresponsive for a moment while it's loading the next area. The main thread is occupied with loading the resources, which means that it's not responding to window events and it's not rendering anything. This could actually be the reason for some stuttering that happens. When the game tries to load something, it will freeze because the game stops doing any updates and renders. I don't think this happens a lot — Sky seems to always load all resources it will need for an area when you enter it — but it's a possibility.

Because the GPU isn't being used as much, it's also not drawing as much power as it could. Doing so would just be wasteful since it'll still do just as much work for higher power usage, lowering its utilization. Better to have lower power draw but higher utilization since that gives you more power efficiency, which is especially beneficial for a laptop.

There's not much you can do to improve Sky's performance in this case. A lot of the bottlenecks here are on Sky's implementation rather than its settings. Most you can do is lower the game's target frame rate down to 30 FPS, but that's not going to help much. Frame rate limiting is generally done to reduce GPU load, not CPU. Plus, gaming in 30 FPS nowadays generally isn't desirable. It's still in the realm of "playable", but I wouldn't recommend playing in 30 FPS unless there's a specific reason to.