Isn't "below mediocre" the same thing as "bad"?
'mediocre' is the median between good and bad. So if it's below the median that makes it bad, right? ;P
Aw crap, I just got called out!
Well I tend to be less harsh towards games. It's less than mediocre but it's definitely not "horrible" as you said. There are much, much worse RPGs from the NES era, Hoshi wo Miru Hito comes to mind. Though how easy the game is contributes to why I think the game is not terrible. To be honest, I think Mystic Quest is much better than hardcore RPGs like Dragon Warrior NES where you have to grind for countless hours just to buy a weapon. Though I will recognize that Mystic Quest's engine is broken to favor the player and is not even closely superior to the Dragon Quest NES's, Mystic Quest at least has a sense of progression that Dragon Warrior NES lacks. In addition, Mystic Quest has little gimmicks where the player can interact with the environment.
That said, I would totally play Mystic Quest on the SNES over Dragon Warrior NES any day.