Ad traffic coming from mobile -> Seeking signup best practices for desktop app!

Context: Just started running some paid Ads with FB (across multiple placements), about 90% of the clicks are coming from mobile devices (to no surprise). The SaaS in which users are being prompted to sign up for is predominately a desktop application...

Of course I could change the Ad placement strategy to only appear on desktop devices but I don't want to sacrifice all the mobile traffic users.

My fear is that users will sign up on mobile (landing page is mobile optimized) and then get into the app on mobile, have a bad experience, and never return lol. This is for financial planning software so it doesn't really make sense to invest in a really good mobile experience at this point...

Question: Are there any best practices/examples of SaaS in similar situations? Maybe you've dealt with this yourself? Tips/tricks, feedback wanted!

Thanks in advance,