Indeed the slider component display images as they are without any adaptation, reason behind this is quality of images which are badly affected when configured in Cover mode inside a slider.

Here is a workaround: When you are in the pagebuilder, go to mobile responsive mode and there you can duplicate your slider, hide the first one on mobile and configure the second one to be visible only on mobile. That way you can upload alternative images for your mobile slider.

More details about component visibility in responsive here: https://support.appdrag.com/doc/Show-and-Hide-elements-for-some-devices

Another option is to hide the slider on mobile and show a Hero section instead (same trick than above but simpler)