Ipad as display for mac mini

You really didn't read the link.

The wire is only for charging, Sidecar use Wi-Fi connection.

Here for your convenient:

Extend or mirror your Mac desktop with Sidecar

Follow these steps on a Mac and iPad that meet the Sidecar system requirements. You can use Sidecar wirelessly, but to keep your iPad charged during use, connect it directly to your Mac with the USB charge cable that came with your iPad.

Did you read the link I've posted?

Here for your convenient:

Sidecar system requirements

Sidecar requires a compatible Mac using macOS Catalina or later and a compatible iPad using iPadOS 13 or later.

• MacBook Pro introduced in 2016 or later
• MacBook introduced in 2016 or later
• MacBook Air introduced in 2018 or later
• iMac introduced in 2017 or later, or iMac (Retina 5K, 27-inch, Late 2015)
• iMac Pro
• Mac mini introduced in 2018 or later
• Mac Pro introduced in 2019

But that is not the answer to my question.

When the Ipad is connected to the mac mini via usb cable is it charging at the same time as used as a monitor?

Yes it answered your question with all the details needed.

With Sidecar, you can use your iPad as a display that extends or mirrors your Mac desktop.

Not as a standalone display.

Read here => Use your iPad as a second display for your Mac with Sidecar - Apple Support

Even I'm using mac mini?

Sidecar system requirements

Sidecar requires a compatible Mac using macOS Catalina or later and a compatible iPad using iPadOS 13 or later.

• MacBook Pro introduced in 2016 or later
• MacBook introduced in 2016 or later
• MacBook Air introduced in 2018 or later
• iMac introduced in 2017 or later, or iMac (Retina 5K, 27-inch, Late 2015)
• iMac Pro
• Mac mini introduced in 2018 or later
• Mac Pro introduced in 2019

With Sidecar, you can use your iPad as a display that extends or mirrors your Mac desktop.

Read here => Use your iPad as a second display for your Mac with Sidecar - Apple Support