Yes it is. But it's more like the final straw that wasn't immediately countered by something else really bad on Trump. If it hadn't come up at that time, she probably would've just clinched Arizona, Georgia, Florida, and NC to offset any Midwest losses.
However, her victory would've been utterly marred because of the other unfavorables. At best, she'd would've ended up like Bush Sr and just a had mediocre presidency in frustrating gridlock...only to be taken out in 2020, a loss far costlier than this.
At worst, she'd end up like Richard Nixon [instead she's more like Richard Nixon was to Democrats before Watergate] and actually do something awful out of sheer paranoia.
Well, Clinton run a terrible campaign and lost because of many other reasons.
If she was running a great campaign, the Comey letter would have reduced her margin, but she would have won regardless.
The question: Was Clinton barely winning and was Comey letter just enough to tilt the election to Trump?
The answer is obviously yes.
