If a coin has been 'improved' by a careful act of cleaning, then I see nothing wrong with that, in fact I would applaud it. There are a mix of positive actions, such as gentle washing, extremely brief dipping to remove ugly tarnish, the application of surgical spirit to remove surface grime, acetone, or the long term soaking in olive oil to remove corrosion, etc. They are all cleaning, but can be done with care and attention.
However, if a coin LOOKS cleaned - polished, unnaturally toned by dipping, or covered with hairlines - then that's a Bad Thing and will negatively affect the coin's value.