Is the transfer time from PostgreSQL to the application optimisable?
That time is spent by pgAdmin to pack and render data and is not the time spent by Postgres to complete the query execution. Why are you fetching 250.000 rows into pgAdmin? If you need to export the table to a plain-text file (like a CSV with header) you can execute this query:

COPY (
  SELECT 
    mobile_state_message.latitude, 
    mobile_state_message.longitude
  FROM 
    public.mobile_state_message
  WHERE 
    topic_name = 'MyCar'
)
TO '/path/to/file.csv' WITH (FORMAT CSV, HEADER);


Remember that /path/to/ must be owned by postgres user, so run chown -R postgres /path/to/.

This way you can use wathever to analyze the table (if you need to).