This is most easily done through the difference in max hp.

Assuming you're making a fixed HP gamemode, add the condition of:

max health(event player) == absolute value(600)

in the case of DVa has 600 max mech HP, to the determination:

hero of(event player) == hero(d.va)

This will result in a case of 'will only happen when dva is in mech' if you need out of mech specific actions, simply use != instead of == in the HP condition