I see that you used the Value mapping but then not used the Regex but instead the default Value one.
You need to delete it and then click “+ Add a new mapping” and there select Regex
I attached a screencast (open it in a new window as will be easier to zoom in and view more clearly) and it works fine with mock data in the same format as uptime
One last trick is to create a new panel and apply the above if does not work in the existing one.
I agree, I saw some post on the Zabbix forum related to uptime formatting.
Therefore would recommend asking in there as if this can be done on the query level then its more efficient and less network bandwidth consuming for Grafana to pull a complete dataset.
Try to use either Regex via Transformation OR use Value Mapping (as posted here) but instead of using Regex, use Value and type day and match it with blank or space so only number will appear on the Panel !!