I think as long as you only have the one then I wouldn't syn it. Syning pureed/mashed/cooked fruit is only because you tend to eat more of it once it's been messed with and hence consume more calories than you otherwise would. As long as you stick to the one banana then I wouldn't syn it.
Muller lights are free food but if you were to eat 20 a day on top of your usual food you would gain weight, it's all about moderation.
If you've got some of the lolly holder things, you can cut a large banana in half, then stick the holder / stick in the cut end, then wrap in film or foil to freeze. This way you get 2 lollies for each banana, and no freezer burn on your fingers.