Hi!
Basically I encountered the same issue, so I just post here to say I think it would be a good and logic feature.
What’s more, it currently looks a bit tricky, since GDevelop allows me to select several instances and edit their instances variables in the properties panel, but it actually doesn’t do it.
Depending on the cases there might be workarounds, but they are limited:
The solution of using objects or groups to massively modify instances variables only works if all instances of an object or a group are concerned. However one might want to edit object variable for some instances only in an object/group, and not all of them.
In this topic Change value to variables to multiple instances in editor, Gluk proposed a solution of having events at the beginning of the scene to edit these instances variables at the beginning of runtime.
However this works only if the desired instances have common points that help to massively filter them to edit their object variables (for example instances’ position between x and y boundaries, instances on a same layer, all instances close to another object, etc.)