Dolphin file manager in Kubuntu (Tested this in Kubuntu 21.04) does not ship with Mount ISO option in the right click menu, but with the help of the dolphin-plugins package we can add this and many other features.

In order to get this working we need to install this package.

Enter the command in Konsole to install the package.

sudo apt install dolphin-plugins

Here is a little information below about the package (run apt info dolphin-plugins to see this info in a Konsole window).

Package: dolphin-plugins
Version: 4:20.12.3-0ubuntu1
Priority: optional
Section: universe/devel
Origin: Ubuntu
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Debian/Kubuntu Qt/KDE Maintainers <debian-qt-kde@lists.debian.org>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 2,342 kB
Depends: dolphin, kio, libc6 (>= 2.14), libdolphinvcs5 (>= 4:20.12.3~), libkf5completion5 (>= 4.97.0), libkf5configcore5 (>= 4.98.0), libkf5configgui5 (>= 4.97.0), libkf5coreaddons5 (>= 5.0.0~), libkf5i18n5 (>= 5.0.0~), libkf5jobwidgets5 (>= 5.70.0), libkf5kiocore5 (>= 5.44.0), libkf5kiogui5 (>= 5.69.0), libkf5kiowidgets5 (>= 5.0.0~), libkf5service-bin, libkf5service5 (>= 4.96.0), libkf5solid5 (>= 5.0.0~), libkf5texteditor5 (>= 5.9.0), libkf5textwidgets5 (>= 5.0.0~), libkf5widgetsaddons5 (>= 4.96.0), libkf5xmlgui5 (>= 5.0.0~), libqt5core5a (>= 5.15.1), libqt5dbus5 (>= 5.14.1), libqt5gui5 (>= 5.8.0~) | libqt5gui5-gles (>= 5.8.0~), libqt5network5 (>= 5.8.0~), libqt5widgets5 (>= 5.8.0~), libstdc++6 (>= 4.1.1)
Enhances: dolphin
Homepage: https://apps.kde.org/en/dolphin_plugins
Download-Size: 323 kB
APT-Sources: http://us.archive.ubuntu.com/ubuntu hirsute/universe amd64 Packages
Description: plugins for Dolphin
 This package contains plugins for Dolphin that enhance its functionalities.
 .
 There are plugins that offer integration with the following version control
 systems:
 .
  * Bzr
  * Git
  * Mercurial
  * Subversion
 .
 Also, there are plugins to offer integration with the following services:
 .
  * Dropbox
 .
 Miscellaneous plugins:
 .
  * Mount ISO
 .
 This package is part of the KDE Software Development Kit module.

Once you have the package installed, it will add the Mount ISO option in the right click menu of Dolphin.

Thank you for reading.