Would really appreciate this feature as well.

I am moving over from Tiller and currently maintain a separate spreadsheet with notes.

Would love to no longer have to do that.

Would really appreciate this feature, including the ability to set the note on a category level, or monthly budget level.

This would be super helpful! I came here to add it but happy to see it's already an idea. Notes for transactions are my most appreciated feature, and I can see notes for budgets being just as helpful