Unfortunately Covid-19 has affected mostly those most in need of help. Care homes need to try and protect their residents as much as possible and strict rules need to be in place. The rules change regularly because the threats from Covid change as we learn more. In my experience care homes try to do their best to provide activities and stimulation to their residents when visits are not possible.
You could try asking the care home what it is they can provide your mum, and contact them regularly to ensure you are up to date with the latest rules.
Hopefully it won’t be much longer before we can get back to visiting safely.