1st off, Wall Level is a very big range. So knowing their exact AP/SS & Dura numbers is important.
Anyway, without knowing what their AP/SS & Dura are scaling to &/or calc'd at, I'd assume Shrek. Quoting Karkat Vantas's own page:
Weaknesses: Harbors an absurd amount of self loathing, Possibly the most irritable person on Alternia, Often feels so inferior and helpless that he is unable to do anything, Little actual combat experience
So we have someone irritable, with inferiority complexes, & "little actual combat experience" vs someone who's actually fought a bit, tossing armored knights into the barrels, fighting through small armies....
Shrek is a brute, & not skilled, but he's definitely got more combat experience. His biggest risks are fooling around -according to his own weaknesses section, apparently.- & maybe getting distracted by his 4th Wall Awareness. ("What?! There are people watching us fight?! What kind of weird nerds are these people?! Go away!")
I dunno if 4th wall awareness actually has a use here.
MAYBE there's a risk of his fooling around getting to Karkat enough that Karkat actually breaks through his inferiority complex, but I'm still unsure & doubtful he has the skill or experience to outfight even a brute like Shrek. Especially if he starts trying his best late into the match.
Also, nothing indicates his statistics would change if Shrek riled him up enough anyway.
The best advantages I can say Karkat has are maybe his intelligence if he applies it & his extended melee range with his sickles. How useful his intelligence partially depends on the place of battle, however.
Also, I'd bet Shrek's High Stamina rating beats Karkat's....
....Wait, what the heck?
Why doesn't Karkat Vantas HAVE a Stamina rating on his profile?!?!?!
(Anyway, for now, voting Shrek via experience, possibly higher combat skill, High Difficulty. Also, would like to know AP, SS & Dura values for both, & Stamina for Karkat.)