Can an iPad mini 2021 support external monitor via USB C with AV adapter?

Yes, iPad mini 2021 now features a USB-C port for up to 5Gbps data transfer, which is 10x faster than the previous generation, and connects to a vast ecosystem of USB-C accessories, including cameras and external storage and displays up to 4K. USB-C enables workflows with high-bandwidth input and output for everyday users and creative professionals alike — perfect for a photographer connecting cameras when shooting on location, or a doctor conducting an ultrasound remotely.

Yes. Depending upon your monitor, the iPad mini6 USB-C connector can be used, with an adapter, to connect to an external monitor. HDMI and VGA monitor connections are supported.

Various adapters are available from Apple and third-party manufacturers. Here is the Apple USB-C Digital AV Multiport Adapter:

https://store.apple.com/uk/xc/product/MUF82ZM/A