Could I2C.readFrom() be made to return a Uint8?

As @MaBe says, you can convert it to a normal array as above.

Also, if you just want to display it as a string, you could use x.join(",") to create a string by sticking all elements together with , inbetween.

The reason type info is displayed on the console is it's very useful as a debugging tool. Uint8Array isn't the same as an array at all. You can imaging the following scenario:

>x
=[ 113, 39 ]
>x[0]=-1
=255
>x
=[255, 39]

That'd be really confusing if you weren't aware that x was a Uint8Array.