You must run a macro to do this, there are several formats you can change in a comment's font:
Code:
Sub ChangeCommentTextFormat()
For Each Coment In ActiveSheet.Comments
With Coment.Shape.TextFrame.Characters.Font
.Name = "Arial Bold" '"Times New Roman"
.Size = 11
.Bold = False
.ColorIndex = 0
End With
Next Coment
End Sub