I've been following Dr. Applesauce for Funko-related things for some time now.
I think I learned how to re-morph a Funko pop because of him. It's if your pop is not standing properly, you can alternate between hot and cold water.
Just check out his Youtube.
Anyway, in this blog, he is talking about the idea of grading Funko pops.
I am a card collector and I only have one graded card and I don't really care for it as much as my raw cards. If you live in the Philippines, sending cards to faraway places isn't as easy. Also, I like looking at my cards. It's difficult enough to handle cards with chunky toploaders. What more on slabs?
So for me, grading Funko pops is worse.
Aside from the mishandling of pops from port to port, now people have to scrutinize the edges, printing, and dents of the boxes. Whether you are an on-box or out-of-box collector, if your collection can fill up a room, then adding more layers to a box is going to be a chore in storage.
I guess this is whether or not you are a Funko Pop collector or an investor. Regardless, I don't think adding a PSA slab will make a pop more important. Yes, it would look good but there are too many intangibles.