Personally, I think the mycelial network is a fun take on the same silly Trek-science of subspace. When I was a kid, I thought subspace was a real thing and that the science of pushing a ship into the subspace plane with a warp core was totally believable. The
mycelial network and the spore drive seem like a natural progression of the same idea, but it has the added fun of an organic component. I really enjoy the idea of living and corporeal fungus has part of its structure entangled with the quantum realm that its root system can reach into the depths of subspace. The technobabble explanation of it in episode 5 (or was it episode 4?), well I totally geek out with technobabble in general, but it made sense in-universe building off already established Trek-science ideas. It was so much fun to hear Stamets talking about it.
It also retroactively makes characters like The Traveler more believable, in that we can kinda say there is precedent for organic beings to be tied into subspace purely by genetic makeup. An idea that regular everyday beings can, in fact, evolve to a 'higher plane of existence' that sci-fi loves to posit. The Traveler has never been a completely believable character through scrutiny, but he was the type of 'evolved' alien that we were use to back in the day. Now, we get more sciencey sounding explanations, but they're still the same magic.
The made-up realm of subspace allowed us to believe that FTL drives were possible, at a time when we were told that FTL is absolutely without-a-doubt impossible. Nowadays we accept that it
might be possible, but the engineering and power to do so is outside our current grasp. So today we have a made-up living organism that exists in our dimension but is entangled with the made-up realm we learned to accept, allowing for even faster than FTL drives possible. Tomorrow we might learn that there is a type of mushroom that curiously has root systems that are influenced by some kind of weird quantum entanglement effect. It's totally unlikely... but quantum physics is that weird science that says anything can happen for no reason whatsoever :P
Trek-science has always been fun and this new take on it is no different for me.