While the iOS/iPadOS Mail App supports use a text signatures, you cannot add graphics or images.
You are not the first to encounter or complain of this issue - and are unlikely to be the last. Apple does invite submission of comments and feature requests via its Product Feedback portal. For iPad and iPadOS:
Feedback - iPad - Apple
If you would like to see new or extended functionality in a future version of iOS/iPadOS, this would be the appropriate channel though which to make your request. Every submitted request counts - as this may be the metric that triggers inclusion of new features that interest Apple’s customers.