How can I make it look more uniform?

This may grow on you,. If I hadn't read what you put in your comments I wouldn't have known what you wanted to do something with that particular portion of the tile. You could put a shelf there to match your cabinets and put salt and paper shakers, a mortar and pestle etc. on the shelf. This would be practical and block most of the other tile. Another idea would be to place some sort of decorative brackets on the side of the cabinets to distract attention from the tiles you don't like.

you can paint over tiles and I will include how to do that for you - our you could add a floating rustic shelf to use for spices, etc that will make it less noticeable

https://brepurposed.porch.com/2017/09/21/paint-tile-backsplash/

https://www.classyclutter.net/install-floating-shelves/

Try a tinted “white wash” to reduce the brightness of the darker and non matching tiles. Use a very light tan to coordinate with your tiles. Use a heavy hand with the tan to lighten the area up. Look at the whitewash item on this page https://www.heirloomtraditionspaint.com/finishing/ Add a little tan to this color to “bleach” and beige up the mismatches. There are a lot of videos and pictures from this site.

Looks like darker grout was used....if you could take out or paint the grout to match the lower tiles, I think that would look better. After that I think a shelf where the new tile is would look fine....you could even put a light or a mirror on that portion.

It doesn't look that bad. I wouldn't say mismatched but too busy to the eye. You can use ceramic craft paints from Hobby Lobby or Michael's to paint the tiles a more uniform color or colors.

I think the same color grout would make it all look more uniform. You can buy a paint intended for grout that would do the job.

Painting the grout lines to match will be a major improvement.