The Activity Stream on an Account has many features. Below the filter, search, and sort options is a text field where you can add a new Note without opening an additional overlay:
Click the field to begin typing your Note. This will reveal a limited status selection - open or closed. Click Add when you are finished. When the overlay is refreshed, you will see a new Ticket in the Activity Stream:
"Note" Ticket types will appear shaded in yellow, the same as the text field, so you can tell them apart from other Ticket types easily.