The PageRank is not relevant anymore, you can read more about this on Wikipedia.
Aside from that, this question is not specific to Kirby as Kirby does not include any SEO features on its own.
Like @lukasbestle said. Don’t trust pagerank. Trust traffic, conversions and positions.
About the Kirby side of it…
It does not matter if you use Kirby or some other CMS. What matters is the HTML, speed, links and header status.
If you moved from another CMS, check that the links match the page exactly. For example WordPress uses ending slash, Kirby does not. Details like that could matter when it comes to inlinks.