Como might be considered a weak club by you, but what they did to Napoli was the same thing they did to Milan, Fiorentina and Juventus. Como got all three points from these four strong clubs including Napoli, so I am not surprised. However, when you underestimate your opponents, you will end up been defeated. Napoli wasn't serious in this match, and it has cost them losing their position to Inter.
Underestimating the opponent and not being serious in the match is very fatal for any strong team in any season. Because the team that is considered weak does not underestimate its opponent and they themselves must be quite enthusiastic in playing the match so that all the efforts they have will definitely be used in the match. And the result itself is also quite clear that Napoli lost to Como who had been underestimated before the match started so Napoli had to experience a decline in position in the current Serie A standings.