It's also bad from Hacktoberfest that they reward unmerged PRs. It would be better if they only rewarded PRs that are actually accepted or at least do something meaningful.
I worry the spammy set would then turn into a competition on how to harass maintainers to "merge my PR really quickly please please please!", but maybe.
If the DO team weren't completely self-serving anti-socials, they'd have set it up to spend 5 seconds eyeballing each PR before sending out a t-shirt, and warning people that only good PRs would count, and giving a few examples from past years.
Or just direct newbies to a bunch of volunteer/sample repos dedicated to helping newbies learn the system, and maybe offer a competitive higher tier of prizes for PRs nominated by repo owned and chosen by DO.