It depends on the objective of your campaign. I can answer this question considering two scenarios:
Scenario 1: Sending your PPC traffic to a landing page with form
If you’re planning to use a landing page specifically built for PPC then you can focus on optimizing your landing page to improve your conversions and in this case SEO for the whole site is not required.
Scenario 2: Sending the PPC traffic to your website
In this scenario, if you’re planning to send your PPC traffic to your website then it’s better to do at least the basic SEO for your entire site.
Remember, the performance of your campaign depends on the quality score in which the quality and relevancy of your landing page is taken into account. Without a proper SEO, I doubt that you can create relevancy for your website or landing page. Your PPC campaigns can leverage your optimized website or atleast the pages you’re targeting.
For PPC, whether it’s landing page or a few pages which you’re targeting in your website it’s better to do the basic SEO. It’s better to keep your home clean before inviting customers.
Focus on the journey of your customer when he/she lands on your page. Analyze whether you have created the experience which is required to convert them as a customer. Once you think along that line it will clear all your doubts.