Tone is an important thing in a campaign and an inconsistent tone can quickly ruin the experience. A consistent tone does not mean that the tone never changes. It only means that the tone is primarily focused on one feeling. This is one of the first things that needs to be established for a campaign before it begins. The tone is one of the major selling points for any story. Just saying the campaign will be focused on defeating the Galactic Empire doesn’t inform the party of how it’s going to feel. If you tell them that “It’s going to be a series of insanity as the group who may, or may not (Probably not), be prepared to do so topples the great and powerful Empire” it will have a dramatically different feel and appeal than if you were to say “A group of weary travelers and soldiers will try to do what the Jedi order could not and bring down the Empire. Though they stand against the unstoppable might of the Sith they have hope and it might be enough to win the day.” These two descriptions give a dramatically different sense of what the campaign will be even though at their core they are the same campaign. Choosing your tone beforehand will help you structure the rest of the campaign and help inform any of your necessary improvisations and descriptions.
Not adhering to a chosen tone can cause a rift with players. If the players were sold on the latter idea above and expect an epic and intense campaign and then you decide the bounty hunter accidentally buys nerf darts instead of poison darts and the AT-ST walkers are running around making chicken sounds then the player will be rather upset. That is not the campaign that they were advertised. They agreed to play and made their character for an epic story of strife and accomplishment, not Monty Python’s Empire Strikes Back. So it’s good to choose a tone that you feel comfortable running and improvising within. This doesn’t mean you can’t have light-hearted moments in a serious campaign or dark moments in a funny one. It’s all about balance though. Shows like Avatar the Last Airbender are very good balances between Epic and Comedy as are many of the Marvel movies. While Avatar has many comedic moments it always comes back to its heavier themes and more serious and epic tones. The same with Star Wars where funny moments such as Poe Dameron trikcing General Hux with a bad connection are brief respites from the intensity of Kylo and Rey’s interactions and the overall plight of the Resistance. Films like most of the Marvel movies tend to be more light-hearted and comedic than they are serious. Films like Thor Ragnarok and Guardians of the Galaxy have serious moments among the near non-stop jokes. But they are woven into the story and are rare enough that they don’t betray the tone but instead enhance it. When deciding the tone for your campaign it’s best to watch a few films and maybe some shows in the same genre and with the same tone you’re considering to get a sense of what works and what doesn’t and what you can take to incorporate into your own campaign. The campaign’s tone is ultimately up to the narrator but it is important to ensure that it’s one the players are interested in and excited for.