Maybe.
Exit polls suggest that late-breaking voters flipped WI, MI, PA, and FL to Trump, but we don't know how much of this was due to the Comey letter - I think another factor was Republican voters coming home.
According to CNN's exit polls, the H-T split was:
| when | Nation | WI | MI | PA | FL |
| last few days | 43-43 | 30-57 | NA | 37-53 | 46-48 |
| last week | 41-49 | NA | NA | NA | NA |
| Oct | 37-51 | 39-54 | 35-55 | 54-41 | 46-47 |
| Sept | 46-48 | 40-50 | NA | 42-56 | 46-52 |
| earlier | 52-45 | 52-46 | 52-46 | 48-51 | 50-48 |
The Comey letter was Oct 28, 9 days before the election, and the second letter clearing her was 2 days before the election; thus, reflected closest in the first two rows. So, Hillary's dive was in October before the letter, and she actually improved a bit after the first letter nationally (no state data). It seems to me it was more because of the usual swing of undecideds against the incumbent (which she effectively was).
Of course, with the election being so close in the big swing states, every little thing makes a difference. But its hard to find a Comey effect in the exit polls especially considering Hillary needed to win all 3 of WI/MI/PA.
Another way of looking at things: the same exit polls show that 5% of voters made their decision in the last week (4% in MI). The H-T split would have needed about 17 more points (more in FL) in that period for Hillary to have won all 3 of MI/WI/PA.
Poll
