No, elm-webpack-loader does not uglify code by default (https://github.com/elm-community/elm-webpack-loader/blob/master/index.js#L15). By default webpack will uglify your code, but it will not apply the flags which are optimal for Elm.
You need to use https://github.com/webpack-contrib/uglifyjs-webpack-plugin (at least for webpack 4) with custom config. The problem for me is I am not sure what that config should be