Make selected text in Notes the default style (no custom fonts, colors, bold, underline etc)
Unfortunately there is no clear formatting option or shortcut, however you can copy and paste the formatting of selected text.

For example if you have a note open with normal formatting, even just opening a new note and typing hello, press Command + Option + C to copy the default normal formatting, then select the note you want to clear and press Command + Option + V to paste that formatting over the text (i.e. clear the formatting).

Also as a point of note, typically you only get these issues from copying and pasting from a formatted document. You can avoid this by using Paste and Match Style which is Command + Shift + Option + V

Hope that helps!