Are pollen masks usually effective for preventing allergies leading to asthma?

It varies... ...At least some relief of symptoms is likely, complete resolution is not.
Sometimes. It really depends on which allergens you are trying to avoid. All allergens have different size particles and pollens are generally light and distributed in the air and therefore can get through most filters. Dust mites are larger as are pet danders and thus do not spend much time in the air and can be avoided with some masks. Particularly niosh masks.

Related Questions

What do you do to prevent pollens going inside your lungs to prevent allergy/asthma symptoms?

Hard. You can wear a high efficiency mask, which is hard to do. Some people try to stay indoors in controlled environment. If there is one particular kind of pollen, you might be able to avoid it. Read more...