I mean, the title is entirely accurate. From the Wikipedia page about sleeper hit:

In the entertainment industry, a sleeper hit refers to a film, television series, music release, video game, or other entertainment product that was unpromising on release but eventually became a surprise success. A sleeper hit may have little promotion or lack a successful launch but gradually develops a fandom following media (including social media) attention, which in turn increases its public exposure and public interest in the product.

The movie has had a very much disappointing opening and the sequel looks suddenly in peril. If things continue like this, and it’s wrecked by Wild Robot, it will fail and be a sign of a “franchise in decline” as per the title— exactly because it’s number 10ish in a series that started 40 years ago. If it somehow succeeds from here, it is a sleeper hit per the definition.