Можно так:
function createTable(db) {
    let tableContent = document.getElementById('content');
  
    let maxTextLen = 60; 
    for(let i=0; i<db.length; i++) {
        let row = `<tr>
                        <td>${db[i].name.firstName}</td>
                        <td>${db[i].name.lastName}</td>
                        <td class="about">${ (db[i].about.length > maxTextLen) ? db[i].about.substring(0, maxTextLen) + ' ...' : db[i].about }</td>
                        <td>${db[i].eyeColor}</td>
                   </tr>   
                 `
        tableContent.innerHTML += row;
    }
}
createTable(dbObj);
Нужно убрать стили:
.about{
        max-width: 10px;
        white-space: nowrap;
        overflow: hidden;
        text-overflow: ellipsis;
    }