Most times whwn mirroring an iPad to any monitor or TV, the image will NOT fill/fit the monitor screen.
Most iPad screens are a ratio of 4:3, like old TVs from way back.
So, that is the screen mirror ratio that will be mirrored to any modern TV or monitor.
The iPad screen and even most TV mirror apps will NOT fill a modern video monitor or today's flat screen TVs with a much linget length to height ratio.
Sorry.
Hopefully, Apple will fix this with the new 2021 iPad Pro models and any up and coming subsequent new iPads and with the yet to be announced iPadOS 15.