I put mashed bananas in the fridge in a bowl for a couple days and they turned dark brown on the top. Are they still good after that?
Cut or mashed bananas, like many fruit, will turn brown after a very short time, a few minutes should show first discoloration and after a few hours, the surface will be brown. This is primarily a matter of aesthetics, because the fruit looks less appealing.
As far as food safety is concerned, it depends on your definition of “a couple of days”. The rule of thumb for storing whole bananas in the fridge is 2-3 days, but you are dealing with mashed fruit, which means a much larger surface area and some broken cell walls. So one or two days should be ok. But if you notice any signs of spoilage like discoloration apart from the aforementioned browning, visible mold or a strange smell (acidic, boozy, or just “off”), discard them regardless. The general rule is always:
When in doubt, throw it out.
No I'd think not. That brown-ing is a sign of rot and the skin is supposed to help prevent this. Now that the bananas have rot outside of their skin I don't feel there's much that can be done at this point. Not only will these mashed bananas taste funny, they may also give you a stomachache or more. Normally I'd say that you could just cut off a portion of the banana that wasn't brown, but that would be with the skin on.