It depends what you want to do:
I don't think Postgresapp comes packaged with address_standardizer or pgRouting, so no you can't install those via CREATE EXTENSION and really can't install them at all unless you compile them.
pgRouting is for network routing, so if you have no need for building trip navigation, you probably don't need it. Most people start with PostGIS before they start using pgRouting.
address_Standardizer: is for address standardization and was a separate module, that windows stackbuilder ships. No other distro to my knowledge ships it. This was merged into the PostGIS code base in PostGIS 2.2, so when 2.2 is released, most distributions should carry it. Again don't need this for normal PostGIS functioning and is used as an alternative address normalizer for tiger geocoder.
So the PostGIS related extensions you can install with PostgresApp are
postgis
postgis_topology
postgis_tiger_geocoder
Postgresapp has the other usual PostgreSQL extensions like hstore etc. And it has PL/V8 which is cool. To see what you have available (and already installed) to you as extensions run the query:
SELECT * FROM pg_available_extensions;