From a performance perspective, applying too much thermal paste isn’t really an issue. As long as it’s evenly spread beneath the contact plate of the cooler, it should get the job done the same as if you’d applied the perfect pea-sized amount.
However, you certainly don’t want to be dealing with a bunch of excess paste if you can avoid it. If you’ve applied far too much thermal paste, once you refit the cooler, the excess will be squeezed out over the edges of the CPU or GPU baseplate.
If you’re using a non-conductive thermal paste, your biggest worry is making a mess of your expensive hardware. On the other hand, if you’re using a high-performance conductive paste, there’s a chance that spilling some over the edges of your baseplate may cause it to short nearby components, which can cause irreversible damage to your GPU.
For these reasons, it’s best to be as accurate with your application as possible.