Just save this as karma.py and run it with Python 3.6 or higher.

import requests
import math

INSTANCE_URL = "https://feddit.de"
TARGET_USER = "ENTER_YOUR_USERNAME_HERE"

LIMIT_PER_PAGE = 50

l = Lemmy(INSTANCE_URL)

res = requests.get(f"{INSTANCE_URL}/api/v3/user?username={TARGET_USER}&limit={LIMIT_PER_PAGE}").json()

totalPostScore = 0
totalCommentScore = 0
page = 1
while len(res["posts"])+len(res["comments"]) > 0:
	totalPostScore += sum([ x["counts"]["score"] for x in res["posts"] ])
	totalCommentScore += sum([ x["counts"]["score"] for x in res["comments"] ])
	
	page += 1
	res = requests.get(f"{INSTANCE_URL}/api/v3/user?username={TARGET_USER}&limit={LIMIT_PER_PAGE}&page={page}").json()

print("Post karma:    ", totalPostScore)
print("Comment karma: ", totalCommentScore)
print("Total karma:   ", totalPostScore+totalCommentScore)