Pixelmon uses the same base shiny rate as the original games (1/4096), and using the dedicated shiny hunting methods we have, you can increase it to higher than the maximum in those (~1/180 vs. ~1/513). Most values involved can also freely be configured in the config files: wiki/Config
Of course the server you're playing on may have also lowered those settings. But you'd have to contact them about that - we have no insight into or influence over individual servers.
and flashes I mean mechanics from the legends of arceus and scarlet/violet. when a large number of identical Pokémon begin to appear sharply in a certain area with an increased chance of shiny
the exact name of this mechanics
Mass Outbreaks