Since park effects are calculated with 50% of the sample size (the home team) being one set of players, they single year effects are highly variable based upon the quality of the team.
The Eastern League had an average OPS of .677 last year. Reading had the highest OPS of .704 and they were not really that good an offensive team. Moniak at 741 really seems someone who should have maybe a 105 wRC+ than 115 and that difference is captured in these park effects. It is still there in Reading. They just did not have the home club to take much advantage of it last year.
I look at Moniak having the same OPS as Cornelius Randolph and come away unimpressed with his offensive stats. He is a year younger so a better offensive prospect, but that is in comparison to someone who is a non-prospect these days. Moniak also is worse on the OBP side of the equation, so that is also not encouraging. He is probably somewhere in the 10-15 range in our system. He probably has the tools to be better than that, but not sure he has the tools to be better than a below average regular.