It's complicated.
Many household electrical goods will draw current when in standby mode and even in the case of PSU's & chargers, when in no load ie there is no device connected or the device is fully charged.
The concept is known as
Electricity Vampires.
Berkeley Labs produced a table a few years ago giving a breakdown of such things typically found in a home.
https://standby.lbl.gov/data/summary-table/In the case of PSU/Chargers, there have been two distinct categories over the years.
The older and far more bulky chargers or
Linear PSU's employ a transformer design and generally the greater the output power requirements, the larger the transformer and the box to house it, let alone weight and increased cost etc. These can suck quite a lot if left connected even with no load on the output present and their efficiency is not too good, ie a 300watt output will require perhaps as much as 500watts of input power.
These have largely been superceded by
Switch Mode PSU's which use an entirely different mechanism to convert typical AC mains down to consumer device level DC. (Warning. The link does contain a bit of technical spoddery)
The attraction is they are much easier to produce, use less copper as the few transformers/chokes inside are very small, and they can be very compact in relation to the output power they can deliver and their efficiency is far greater.
They have gone under many iterations in design and sophistication over the years, the early ones were notorious for producing all kinds electrical interference and being unreliable when no load was connected, ie they would blow up quite easily, but these days, provided they are built and tested to appropriate EU or US standards, they are very good.
Nevertheless, even with no load connected to them, they still draw a current when plugged in although the newer Apple chargers are meant to reduce even this significantly, but at a cost in component complexity and thus price to the consumer.
On their own these days, the amount of power modern Switched Mode PSU's consume is likely insignificant to a typical household bill, but multiply it by the number of say mobile chargers left plugged in throughout the UK (or World) then it starts to become significant, especially if you are trying to plan future National Electricity generation needs.
So I would agree with your Mum in that it's likely still good practice, and ethically correct, to unplug such things when nothing is connected to them, or if you have several, have them all plugged into a power strip that can then be turned off at the wall socket.
