If a snippet is created by pasting text into your compose window, Gmail will keep all formatting that was applied to the text before you pasted it. The way your email appears while creating your snippet is exactly how your recipient will receive the message.
Correct bad formatting
Copy the text of the snippet
Paste the plain-text version into a new draft. By default, this is ⌘+Shift+V on Mac and Ctrl+Shift+V on Windows
Format the text to how you'd like the snippet to appear
Click the snippet button in the toolbar and select Create New Snippet from Current Message.
Remove all formatting after pasting
Paste the text of your snippet into a new draft.
Highlight the text.
Click the Formatting options button in the compose window toolbar and select the crossed out T option at the far right end of the formatting toolbar.