I've been watching the anime...and noticed that the monsters have the same eye color as th...

I noticed that too! The difference between a hero and a villain is success...and time. Given a long enough time, killing enough people to obscure their origin and welding enough power a demon in service to an Evil God can simply become a God. Kill off their allies to "protect humanity" serves the purpose of concealing its origins and eliminating the competition. Then humans start to worship the new God feeding it power of their faith. Everything about her should have been suspicious...and where are the other God/desses? Maybe they banned together to get rid of their rivals then turned on each other the last one standing gets to be the highest God by default. There are so many different ways she could have captured the world in her hands then every so often a "demon lord" spawns so humanity ALWAYS needs her presence and never forgets to be afraid. To never forget that they are safe because of her...gifts. The tribe at the end of the world are probably the only ones who have an idea of the truth or at least know something that keeps her at bay. So she ignores them not out of just fear but necessity.

And also...was Mimori and his classmates are the FIRST heroes that were summoned from Earth?

Doubtful. I'm fairly certain I saw some warriors weilding katana and Sougo's naginata is a classic Asiatic inspired weapon. The weapons most used seem to have a European descent like the halberd. There's obviously the possibility of a culture mimicking East Asian cultures in that world but there so many things that are happening in the background I that it feels planned. Heck even a few of the clothes have an Asiatic esthetic

And I've been thinking, since the ENTIRE class-including the teacher- were summoned all at once, isn't it considered kidnapping? Or does time flow differently between universes?

I am of two minds on this. If time is flowing it would be neat for it to Rip Van Winkle so it's moving at the same speed. They would get to come back to a completely different spot in the world then they left. Which is appealing. But I could see them playing the time stopped or they "died" category. Even them being summoned BUT their minds moving rather than their bodies. They'd all come back with so much more life experience they could never really be the teens they were. The classmates who died having like a heart attack in the real world would be a different kind of tragic. I think the "kidnapping" won't be a factor because the Gods probably don't interfere with each other--or rather can't. Then again Visis is probably impersonating a God using old knowledge of magic and no one can say otherwise because she's 'protected by her power as a Goddess'. A technology so far advance from the present knowledge is no different than magic in the eyes of society at the time.

Good point...

And I noticed that the entire class were completely dysfunctional, does that mean that in a scenario such as this made them worse?

It's already been explained the goddess created the human faced monsters

We understand that but WarriorNun was pointing out the Goddess herself has yellow eyes. I was saying that it could be a possibility that she BECAME known as the goddess by 1-eliminating anyone who knew she was not a celestial entity, 2-by having some powers far beyond what is recognized as normal/standard for people of this world, 3-when someone comes along saves a group of people with skills far above their understanding it often leads to them being worshipped and 4-if enough time passes from the "savior" helping the people they have effectively re-written history in their favor saying they are a God. So basically Visis made herself an object of worship by killing anyone who knew she wasn't then convinced the world that it has always been her as a God/dess. She's a human faced monster that kept her pretty face and has been manipulating an entire society

So, is she a former human faced monster that gained both sentience AND intelligence...or the progenitor?

Or worse...the Mother?

See that I'm not totally sure of. The mother of all monsters Echidna does show up in some Manga.