You don't have too much of a choice here. That's the component and it's not very likely to be improved soon.
If the website you are trying to display is yours, you can edit the html file to make the browser not show the zoom in or out buttons (not "thingies").
Search on the forum about that and you will find how to do it.
Another option is to use any of the other App Inventor distros, like AppyBuilder, which provides a property on the webviewer to hide the zoom buttons.
If the website is not yours and it still shows bad in the webviewer, then you will need to check if you are showing the mobile version of it, may be it's not.
As a last resource, and if the website has one, you may want to use an API to send and receive information from that website instead of showing it inside your app.