We're now a few months post-winter holiday. That's plenty of time to see if customers acquired back then end up becoming repeat customers or if they are stuck as new customers.
It's common for holiday-acquired customers to buy and then disappear. It's rare for them to stick around and make a second purchase. If they don't make that second purchase through, you're out all the resources you used to acquire them in the first place.
To figure out how much of an impact one-time holiday customers have, you'd need to slice up Shopify's customer and order reports to see how many placed a second order vs only the first order (stay away from Shopify's "Repeat/Returning" metric, it has a bug with how it counts).
Or you can use the Cohort Report in Repeat Customer Insights as it automatically segments holiday customers into their purchase month (and every other customer too). Drilling down into the November and December cohorts will make them easy to compare to regular months.
Start noticing seasonal spikes in customer behavior
Cohort reports let you detect seasonal spikes as well as long-term retention cycles. Repeat Customer Insights can automatically create cohorts for your Shopify store, going back to your very first order.