The first time you get your character in Smash in any way, shape, or form, it's a miracle. People are ecstatic over seeing them manage to make it, like the popular reception Shovel Knight's Assist Trophy got, even if it was bittersweet to those who wanted him in as a Fighter. They get a model, abilities, sometimes even voice acting. It's a sight to behold to get a taste of what the Smash team thinks of your character. Bomberman is an outlier, though he may have come as a one-two punch.
The second time... not so much. At that point the magic has worn off, and especially since many continue to believe that an Assist Trophy doesn't help for DLC due to the Waluigi statement back during Smash 4, they become dreaded as the "Snowglobe" that serves only to make your character a loser to be mocked by fans of the winners. The most negatively received Assist Trophies in Ultimate were Waluigi, Ashley, Isaac, Shadow and Skull Kid, all of whom were very popular requests, and in the case of the former four had time between Assist Trophy announcements where people could really see them being in.
When it comes to Mii Costumes, so far all of them in Ultimate have either been well liked or considered a consolation prize good enough to not fuss over, with the exception of Rex. But back in Smash 4, people lost their **** over King K. Rool's costume, to the extent of trying to debate if they really meant the end for another prospective newcomer (spoiler: they did, and I'd wager they still do), but he was THE most popular newcomer pick who had a chance. People didn't really hate the others, from what I remember, to any significant extent half as much as K. Rool's (Geno's costume coming back, or even getting a remake, would get a similar reception today, if not worse. But at the time his support was dead and the Mii Costume brought it back to life). So I'd say Mii Costumes are mostly the reverse: people who were already out of the running are appreciated for at least getting a costume, but having to play as a skinned version of your character with a moveset that only fits on a superficial level, if at all, can be really rough, especially when they were popularly speculated. Sans and Cuphead, because they are the "Premiium" costumes, are a notable exception; a music track and not having to look at a Mii's face, which reminds you that this isn't really the character, goes a long way in making them more popular.
In the base game, it's an Assist Trophy, and in DLC it's the Mii Costume. They only work as a "sorry, we couldn't make this character into a fully fledged fighter. Please accept this instead, it's the best we could do" sort of gift once. After that, that's when they become a middle finger, especially when Piranha Plant and Byleth and Corrin, none of whom got the welcome wagon, and whatnot get in while beloved characters are shot dead. That's almost certainly not the intent, they're made with the intent of showing the character love (except for when they're the butt of a joke, like Waluigi or Tingle). But that is, from what I've noticed, the in practice effect.
Spirits are... well... hard to say. But people don't really accept the idea that this is the end of the character. In the case of the post launch spirits, people aren't raging, but they are sad to see them killed off, and as it's currently going on and could happen to anybody then we're in trouble if most of our popular predictions end up in the trash due to Spirit events. Base roster Spirits are debated on if they signal the end of the character or there's a chance, and if so, when the door was open. While I personally believe BR Spirits don't, I can understand the perspective of people who think that they very well may.
Whether or not having nothing is better than having something in game is up to you. By and large, some games like the Henry Stickmin series are just NOT going to be represented whatsoever. There's also the issue of leaker culture that means we're sheep to whatever big leak comes out from insiders or rogues who post text or whatnot. Especially if the Disconfirmation train continues with leakers, Spirits, and Mii Costumes telling us what's in and not. Who is Fighter 6? Even if we get them in April as I believe we will, there's 10 to 13 more weeks of potential Spirit events. If we have to wait until E3, that's 19 weeks of potential Spirits events. That's terrifying, though the number of tourneys we'll get could mitigate that.