I don't know if it was done before, couldn't find anything like that, so I made a tiny script that sums all mertis of a post and displays it before individual merits
Like this:

// ==UserScript==
// @name Bitcointalk Post Merit Sum
// @version 1.0
// @grant none
// @include https://bitcointalk.org/index.php?topic=*
// @run-at document-end
// ==/UserScript==
;[...document.querySelectorAll(".td_headerandpost")].forEach(post => {
try {
let sum = [...post.querySelectorAll(".smalltext i > a")]
.map(e => {
return parseInt(e.nextSibling.textContent.match(/\((.*)\)/)[1])
})
.reduce((acc, e) => acc + e, 0)
if (sum > 0) {
let sumElement = document.createElement("span")
sumElement.textContent = `Total merit: ${sum} | `
post.querySelector(".smalltext i").prepend(sumElement)
}
} catch (e) {
console.error(e)
}
})