Is it appropriate to exit the MEWP platform when working at height?

No, unless a rigorous risk assessment carried out as part of planning the job indicates that this is the safest and most effective means of accessing a particular location, taking into account the availability on site of other more suitable access equipment and the practicability of providing the same within the required timescales for the task to be carried out.

See IPAF's technical guidance E2 on exiting the platform at height and this extract from BS 8460 (the British Standard on “Safe use of MEWPs – Code of practice”) on the subject.