I can see your point. I partially agree and disagree, so I'll go into both sides.
Agreements:
I think that if your not-smash-fan friends come over and are disappointed by the initial roster, it can leave a poor first impression of the game and they will either ask to play something else like Mario Party or Mario Kart, or leave it up to you to have unlocked it prior to their visit or before their next.
The problem with that is that, 1. the characters have been tricky to unlock because of a high skill-ceiling and 2. you being able to beat them is probably an indication that your skill will be way above that of a friend who might've just been introduced to the franchise.
So when they're back and you have the full roster, you will probably body them the entire time and they'll have much less fun than if you all were gradually building up your skill together.
Disagreements:
That's kinda it though. If you all chose to spend an evening or two together to unlock the characters, that would actually be kind of a fun evening. You'd all get used to how it works, you could laugh when you or your friends lose to a silly challenger fight, and celebrate when you win and everyone may start wanting to try the new character together.
It would be kind of like an event by itself, so it would all really take for you to approach it the 'correct' way to make the first few evenings work as a couch multiplayer experience.
That said, past that and when the skill gap isn't an issue, the couch multiplayer has never been better. The sheer options available are staggering and the improvements have been incredible imho. I can't wait 'till I next have my brother come over and we start testing out weird rulesets together.