Changing the Default Cell Comment Formatting
Excel's Insert - Comment command lets you annotate a cell by typing a comment. Once the comment is created, you can change its formatting. To do this, right-click the cell and select Edit Comment from the shortcut menu. When the comment is displayed, click anywhere on its border to select the entire comment object. Now you can use the standard toolbar buttons to change the font, text size, or colors.
To change the default formatting of your cell comments (so you don't have to repeat that process each time), you need to go outside Excel and use the Windows Display Settings dialog box. Excel uses the formatting specified for Windows' ToolTips (the text that appear when your pointer hovers near toolbar buttons).
You can access the Display Settings dialog box from the Control Panel. Or, you can right-click the desktop and choose Properties. Access the Appearance tab of the Display Properties dialog box, and select the ToolTip item. You can then modify the font, the font size, the background color, the foreground color, and the Bold and Italic attributes.
Be aware that this will affect all comments that have not been formatting. If you've already applied formatting to a comment (for example, changed the background color), that comment will not be affected by this change.