I could see why it didn't do as well.
It takes a character who was one of the most normal of the Avengers, and sends him into another universe. The first two Antman movies were caper films.
It started with minor bad publicity by recasting Cassie.
The film mainly sets up Kang as the bad guy in Avengers 5. That narrative is a bit flawed since Kang loses too easily. This could be part of why the second weekend dropoff was so bad.
The special effects were universally critiqued. There are rumblings that Marvel prioritized Black Panther 2.
https://www.vulture.com/2023/02/marv...antumania.html
There may be a version of the movie that is a big hit, where the special effects teams were given more time to make the Quantum Realm special, where there's a narrative that isn't a teaser for the next Avengers film, and where Kang (the casting is well-regarded) finds the balance between not squashing Antman and being tough enough to be an obvious challenge for the Avengers.
Part of it may be that they left too much until the last minute, and with all the Covid delays they already had, they weren't going to hold up Antman, especially since this was seeding Loki Season 2 and the next Avengers.