Currently this is not exposed but could be manually edited if you are familiar with web map JSON. It is on our road map to add this formatting option under the aggregation fields.
Thanks for the workaround. I think it doesn't make any sense to have these numbers as decimals.
@RussRoberts I'm running into this issue with cluster pop-ups in ArcGIS Online, too. Any update on the road map for this? Or better yet, was a solution implemented and I just can't find it?
Edit: Nevermind, I sort of found it. For anyone who comes across this, select the layer, go to the Aggregation panel, open the options for your cluster type (chart), then choose Cluster Fields. From there you'll see dropdowns for each type of feature in your clusters. It seems kind of buggy, though. Once I start messing with anything, the symbols disappear from the map. But if I had one of the pop-ups open already and change one of the cluster fields from Number to String, the pop-up will show the number without the decimal places. Until I close the pop-up and try to reopen it, then the numbers are missing entirely, along with the symbols.
Edit Edit: I got around that specific issue by adding new fields to the Cluster Fields panel, which have expressions identical to the existing ones, and then used those for my pop-ups values. Left the existing cluster fields alone. That seemed to work and removes the decimals, but I have no idea why. Very odd.