Even the menu navigation is not easily accessible. You have to reach the 2nd level of the menu item. And If you are coming from a Windows background, it will even be tricky to find the option in the menu.
First of all the strike thought option is available Under Format -> Fonts.
To make it handier, you can set a keyboard shortcut so that, if you are using Notes more often, then it will be very useful.
Follow these steps to set a keyboard short to strike through in the Mac. It is limited to only the Notes App.
Go to System Preferences, and select Keyboard. Then select Shortcuts Tab
Then select App Shortcuts in the left-hand side pane. Then click on the '+' Icon at the bottom center of the menu.
For APplication select Notes. As this keyboard shortcut is only applicable for Notes, select Notes.
For the Menu title type Strikethrough. This is important and it should match the menu command present in the Notes app. The string should match.
Finally, select the shortcut command you want to use. Make sure this key combination is available not assigned to any other command. I have selected Shift + Command. + X. Since X kind of matches with the action phonetically and pictorially. It makes sense to use X with other key combinations for strike through
