I have heard this expression so many times 'steaming pile of mediocrity'
In short: poop/feces/defecation/garbage/sh**. It's an insult (idiom) used to describe someone or something with comparison to the above. I hope when you heard it it wasn't directed towards you, as the phrase is not commonly used in play.
It's a play on the very insulting and rude "steaming pile of feces", which obviously means something very undesirable or unusably bad.
When something is mediocre it is not great, it is not good; but neither is it terribly bad or unsuitable - it's just an ordinary or typical thing you'd find in the middle of a collection of other ordinary and typical things.
By calling something a "steaming pile of mediocrity", they are specifically calling the thing mediocre (so they can't be quoted as saying it's terrible), but the phrase calls to mind the fecal matter end of the spectrum of quality, implying the thing is actually worse than mediocre.