My mother (93) has just had a mild tia. She is on clopidogrel, lansoprazole, amlodipine, losartan/hydrochlorothiazid, simvastatin, mirtazapine, buprenorphine patches. Should she also take an aspirin daily?

Absolutely NOT. The addition of asa to clopidigrel doubles the risk of intracranial hemorrhage without providing any additional protection.
In principle yes, If the clopidogrel is for cardiac condition . If it was given for the TIA , there must be reason why she was not on asa instead of clopidogrel. At this age , we do not need to be too aggressive so that we will not cause more harm than good . This is not in any way to say that older people need less help.