As a fan of the original manga i was pretty enthusiastic when i first heard about this. Tensei Shitara was and still is able to shine with excellent world building, it´s characters and a story that may come of as generic at first, but is handled so well that you just can´t stop yourself from enjoying it.
Mamono no Kuni on the other hand doesn´t have any of these qualities. It is basicly a lifeless fanfiction with a mary sue author insert character as it´s lead. These words are sadly not an exaggeration. Every character knows her or want to know her, she has a unique
skill and can do whatever the plot needs her to do. Characters of the original manga get thrown in your face an nearly every panel of every chapter so far and are mostly there to tell you how great this new character is. It´s rather frustrating to read.
Another quite irritating point is that this spin off is leaning on the light novel and not the manga and starts months or even years after current arc in the manga. This means it shows characters that haven´t appeard in the manga adaptation and can even spoil some plotpoints for people who haven´t read the novel but do read the manga at the moment.
The artwork is the only thing that is at least tolerable, but nothing to write home about either.
All in all i can´t recommend this spin off to anyone. Just skip this one and save your time for something you can actually enjoy.