Hello,
Onhover tooltip is present to show more detailed information which might not be visible in case if the report canvas in the dashboard is small.
However if you would like via CSS we can disable this on-hover
Open the report in edit mode, click on the meatball icon (three dots in the top-right corner), and select Operations > CSS.
Inject below CSS:
.ant-tooltip-arrow, .ant-tooltip-inner{
display: none !important;
}
You can append the component id also in case of dashboard if you would like to disable this for specific tables in the dashboard. In those cases apply this CSS not at the report level CSS placeholder, but rather at the dashboard level CSS placeholder.

Thank You,
Helical Insight.