Why does Siri answer with uh huh or mmhmm?

Perhaps you may be able to 're-train' Siri after learning how

..and hope that'd be enough, to make it less dull sounding?

see several ideas across helpful sites on internet here

Change Siri settings on iPhone - Apple Support (Asia/English)

//support.apple.com/en-asia/guide/iphone/-iphc28624b81/ios

How to Reset Siri on Your iPhone, iPad, or Mac - makeuseof

https://www.makeuseof.com/reset-siri-on-iphone-ipad-mac/

iPhone User Guide - [TOC] Apple Support (Asia/English)

//support.apple.com/en-asia/guide/iphone/toc

including a mix, some Apple Support, & probably similar stuff(?)

1 https://duckduckgo.com/?q=siri+voice+recognition+reset&t=ftsa&ia=web

2 https://duckduckgo.com/?t=ftsa&q=reset+siri+retrain+voice&ia=web

It happens because you are pausing after giving the "Siri" or "Hey Siri" command. You should just speak in one fluid sentence and should not pause after initiating the command as Siri is capable of processing your request without the pause.

Users who previously had other assistants like Google or Alexa struggle with this the most as they would need to issue the command to "wake up" the assistant first, then give the command. That is not necessary with Siri. You can still give feedback to Apple if you with.

Product Feedback - Apple