It's not share dilution, just a higher number of shares that will be available to trade on the market.
In my personal experience, this doesn't tend to be a big deal at all since 1) the dates are known months or years in advance and market participants prepare accordingly, and 2) the shareholders usually have little interest in dumping all of their shares at once (and Sezzle's ownership is highly concentrated anyway).
I'd be very surprised if this has a meaningful impact on share price.
SZL Price at posting:
$8.21 Sentiment: Buy Disclosure: Held