The residence, domicile and remittance basis comes into effect in this situation.
As your mum is resident, she will need to determine if she is domiciled in the UK (you are deemed domiciled if your have been resident in the UK for 16 year or more).
If she is not domiciled, then the remittance basis comes into effect.
If your mother is resident, but not domiciled and she does not remit the state pension to the UK, then it is not taxable in the UK.
If your mum is resident and deemed domiciled, she will need to declare the state pension on her tax return.
You will find guidance and the test to help determine her situation here:
Residence, domicile and the remittance basis: RDR1
In order to claim resident / non domicile, requires the completion of the residence section of the tax return.