As others have pointed out "malísimo" is indeed closer to "really bad" or "horrible" than just "pretty bad".
However I do think it's weird that Nintendo would threaten to ban someone's online account for this. Regardless of whether or not you think the guy was being an asshole (I don't play MM2, maybe it is really obvious when someone tried hard on a level vs when they're just trolling), there's quite a number of options for Nintendo between saying "don't leave useless negative comments" and "we may ban your entire online account next time". Perhaps Nintendo could implement a better feedback system that protects vulnerable creators more or is more geared toward positive/constructive comments, or I wouldn't even necessarily mind them fully taking away a repeat offender's ability to comment on levels in MM2 without impacting their entire account.
Also I understand where people are coming from with "if you're toxic in one game you're probably toxic in all games", but this is where I think it's important to distinguish between straight up hate/bigotry and more general assholery like this.