I guess the similarity to Superbad is that it is specifically about two teenage friends trying to go to a party, only now they are girls (and also you have one who is short and stocky and one who is taller and slim).
But despite the similarity in the plot setup the rest of the approach is pretty different. Superbad seems like it's more specifically about the nostalgic memories of the people who made it, despite being officially set in the present day (I mean, Van Halen dominates the soundtrack), while Booksmart feels more of the current day.
I'd also like to point out that the movie actually did fine relative to its budget. I think the pattern of release was a mistake, trying to dump it into a lot of theaters in the middle of summer. That created financial expectations the movie couldn't match. Would have been better to have a gradual staggered release like a typical indie, so word of mouth can get around. Even so, $22 million domestic gross for a $6 million budget is fine.