As having a line item for mobile in the media plan is becoming more and more common/required by the management, it is very important to have an end to end mobile experience. It is a very bad user experience if the banners appearing on mobile phones are being used to drive users to a desktop website.
One thing that is for sure is that your site will experience a very high bounce rate (users will leave the site as soon as they land). OR if you still notice that the users are taking actions, be assured that if you update your destination website to a mobile based website, you will see a huge jump in performance.
Driving users to a desktop based application will also take a long time to load the website, which can be frustrating as well.
All the issues mentioned above, will not only lead to bad user experience but can also lead to a negative impact on your brand. Essentially, users click on banners to get more information about your brand but due the website not optimized for the mobile they will not be able to get any information that they are seeking.