Your contractor told you on the front end that the tiles were poor quality and you proceeded anyway. Ripping it out would be on you. Match the grout to the tile and it will be a lot less visible.

yes, the grout will help a lot-use a gray/white that is the same tone (on the grey scale) to help them look more co-hesive

btw-sometimes contractors like to blame the materials to cover their own ineptitude-if this is not the only project you are working on, and you are not happy with his work, you might want to consider interviewing some others. on the other hand, if this is the only project, keep an eye on him, ask questions, and thank him profusely for every good thing he does. anyway-its a nice color/mosaic and your kids will enjoy it!

It's known that floors are not even throughout, but a good installer knows how to work with that. Some contractors want to blame everything on the material and not on their lack of skills. What tile did you use?

I don't remember the brand now, don't have them with me, it was a glazed porcelain tile bought from Rocca tile group, they are called essentials watermelon, they are from Spain. The contractor also did my master bath with a porcelain tile 12x24 on floors, wall, shower, really pretty and that bath is perfection! so that is why I tend to believe he did the best he could with what he had. I also saw the floors prep before installing the mosaic and they looked even to me. I hope with the grout they will look better and of course with the toilet and beautiful vanity that I picked. Just a little disappointed.

Did he say what his concern was? Saying that they are not good quality without giving a clear explanation is not enough. I wouldn't expect something bad from Spain. Usually, the tiles made in China are not good (thickness difference, size, etc)

He said that he could not align perfectly well all the tiles, that if he pushed some so that he can follow the line then a space would open up somewhere else. But that is not what bothers me the most, the issue for me is that the visual effect of the tiles makes the floors not to look flat.

get a level and see if the floor is straight after the tile was installed. If it is, then it is just a visual trick due to the lines not being straight. For the grout, do something that would not accentuate the tiles that much so that you don't see the difference in grout thickness