If you don't want to mash it try putting thickly cut wedges for him/her to pick up (might choke if they're thin) and accompany them with some yogurt, milky weetabix etc.
Yes, it is perfectly ok, so long as they dont just have banana. Like the others have suggested, mix it with something or put something with it. Try and vary it as well so they dont get bored with the same thing all the time.
bananas can make you constipated or give babies very sticky poo - but as long as it doesn't have that effect them it is perfectly fine. I presume that they have something else as well? e.g. cereal or toast.