As strongly suggested by the trailer, it is reportedly a disaster.

Some elements sound entertainingly crazy or inept — apparently it features a "horny dragonfly," "a homicidal squirrel who’s determined to get revenge on the human teenager who shot him in the chest," and "at one point, Dolittle and company are welcomed aboard a new boat by a bearded man who announces “I’m Jeff!” and is never seen or mentioned again." Some reviewers have also revealed what Dolittle does to the dragon he meets, and LOL.

David Erlich's review is entertaining:

Downey, stretching his wings in the role of a rich and tortured genius who wears godawful tinted sunglasses and spends most of his time talking to non-human companions about the ethics of using his abilities to help people, delivers a fidgety, impenetrable performance that makes you wish his Dolittle had done a lot less.
. . .
. . . Marion Cotillard as a French-accented fox named Tutu — who the press notes identify as the leader of “The Fox Resistance,” whatever that is. Most of the creatures in this movie are defined by the anxieties that Dolittle helps them to overcome, but Tutu is apparently involved in an off-screen revolution of some kind. Why not? Justice for the foxes.
. . .
There’s barely enough usable footage to stretch the story past 90 minutes, but the CGI creature effects — rich with personality — manage to find the rare sweet spot between photorealism and exaggeration. The fiasco’s exorbitant price tag suggests precious little threat of a sequel. You could argue that it’s strange to watch a massive Hollywood movie in which every single line seems to have been dubbed after the fact, or you could enjoy the rare opportunity to watch a first-run film that comes with its own “Mystery Science Theater 3000” commentary track. You could lament hiring Stephen Gaghan to direct something that Universal Studios clearly wanted to play like a theme park ride, or you could just sit back and be grateful that you’re not watching “The Lion King.” Some people aren’t suited for the opportunities that are presented to them, but — at least in its own chimeric way — no one can say that “Dolittle” fails to defy expectations.

https://www.indiewire.com/2020/01/dolittle-review-robert-downey-jr-1202203289/