The addition of fat will slow the digestion of the sugars, spacing out the release of insulin rather than igniting it all at once. This is a positive thing, unless it is post-workout (arguably).
There is no "bad" part do doing this, and its a great snack IMO. For a small improvement, it wouldn't be a bad idea to attempt to get in a bit more protein however. A banana plus a spoon of PB is roughly 35g CHO, 4g Protein, and 8g Fat.