When doing a smart group and showing groups that were modified on a date, All the groups that meet that criteria appear, The parent group and the child sub group.
Is there a way of only having the top parent group appearing?
If the subgroups were created in the same timeframe, no it’s not possible to exclude them without other criteria, e.g., a tag, label, etc.
Is there a possibility of filtering by groups that do not have a parent ?
No. There is no location nor a parent criterion available.