Most websites I think get 50–75% of their traffic on mobile, though it can certainly be more or less. Last month I had around 60% mobile traffic on my website. More and more people are browsing on their phones and tablets, so having a responsive site is definitely important!
You can actually check how your site appears on mobile devices using your browser on your computer! That’s how I got the mobile screenshots of the sites I reviewed for the mockups.
If you’re using chrome, go to more tools > developer tools. It should open a menu in the side of your browser. In the top left corner there’s a button that looks like a phone that will say “Toggle device toolbar” if you hover over it. Click that, and your browser will start displaying as a mobile device. Refresh the page once so it can load the mobile version fully, and from there you can actually select which device you want to test it on. It isn’t perfect/as good as testing on a real phone, but it’s a good start!
If my description wasn’t enough (or if you use a different browser), Google search “responsive mode + name of browser” and you should be ale to find instructions with screenshots. I know Firefox has this ability as well, though I’m uncertain about other browsers. There are also browser extensions you can install that make viewing a website’s mobile version even easier.
Depending on the platform you’re using, it might be easier to just make your site responsive, or it might be easier to have a mobile version that small screen sizes redirect to.