First of all, yes this is mostly for the funny.
However. I still believe in this argument, and that this would honestly be the real outcome if this were to go down. If you don't like it, okay.
Ok, so for starters. We are going to say that Goku has LV 20, which is honestly pretty gracious towards Goku. I say this because Chara claims that LV 20 is the "absolute", meaning there is nothing above it. LV 20 is still high, and this means that Sans's Karma would be very high, and would be ravaging Goku for every fraction of a second he is trapped within Sans's attacks. And while yes, Goku is very fast in the real world, this is not the "real world".
Speaking of which...
Because Goku is a human and Sans is a monster, this means that this is going to be an Undertale Style fight. Which already has it's disadvantages for Goku. He's never fought against a monster, and by the time he did learn how to properly learn EVERYTHING, he would already be pretty badly hurt, on top of that, Sans would break all the rules, throwing unpredictable attacks, both during his turn AND Goku's, which would confuse Goku more. Goku is not the brightest (out of battle, anyway), so this would make things very hard for Goku to dodge and survive in this new and foreign kind of combat.
Now, you might be saying "But what about Ultra Instinct?", and that's a pretty fair point. However...
Goku, throughout the manga and the shows, has always had stamina issues with UI. He cannot use it for very long, and is extremely tired afterwards. Sans's has much better control over his stamina, and is able to go for long periods of non-stop attacking, all with stamina to spare once it's over. Not to mention, Sans has abilities like telekinesis, which could basically be the "f*ck you" card that sans could play on top of UI, throwing him in whatever direction he wants.
Finally....
Sans is weak. And that's what makes him strong.
Goku would probably see Sans, his lazy, unserious self, and his incredibly low stats, and go into a fight way overconfident, and the the funny song starts playing... and... well...
You know the rest.
Thanks for your time! Or the "I ain't reading allat" replies. I know what you are.