No, Latias is roaming in the routes in Kanto after you find the doll and give it to the girl.
Try to Get a pokemon with mean look that outspeed latias and use mean loook, a pokemon with the ability shadow tag also works.
To find her try to get in the same grass and use a pokemon that is a higher level than the rest of the grass and use a max repel, the encounter Latias and use mean look as she will escape immediately.
You also need lots of Poke balls preferably ultra balls but dusk balls work good at night use whatever you think fits the job.
and try to catch Latias.
And it's a good idea to save before you encounter it just in case it faints.