I am using Paper-Button but I am facing issue that the button text always gets capitalized instead or normal case.
I do not see any CSS or Javascript property being applied to make it upper case.
As was mentioned in the comments above, the material design spec for buttons specifies that the text should be uppercase, but you can easily override its CSS property:
Inspired by the the CSS style above here is the inline styling for localized Button text transformation -
import {Button} from '@material-ui/core';
// Begin Component Logic
<Button style=\{\{textTransform: 'none'}}>
Hello World
</Button>
// End Component Logic