Can mespirit fly to spear pillar?

Mespirit, being a Legendary Pokémon, can indeed fly. While it's not confirmed if it can fly in the traditional sense like a bird, it can certainly move through the air with ease, as seen in the anime and various games.

So, yes, Mesprit can fly to Spear Pillar. The only obstacle would be its tendency to be elusive and difficult to capture. However, if it wanted to reach Spear Pillar, it would be capable of doing so.