Pippsqueaks is a term used by Pipp Petals to refer to various groups of her social media fans, with most of the group definitions being mutually incompatible:
Zephyr Heights[]
In My Little Pony: A New Generation, Pipp was heard thanking Pippsqueaks while livestreaming to phones and public jumboscreens in Zephyr Heights, appearing to have (and to refer to) a large general audience in that city.
Maretime Bay[]
After moving to Maretime Bay at the beginning of My Little Pony: Tell Your Tale, Pipp began using the term to refer specifically to the 3 fillies Glory, Peach Fizz, and Seashell, with the former 2 presumably also having moved to Maretime Bay as a result of the ponykind reconciliation.
The 3 of them occasionally got special treatment and access to Pipp's production sets, for instance during Pippsqueaks Forever and Fanponies With Glory, Seashell and Peach Fizz.
Other notes[]
Pipp's content viewers in Bridlewood, for instance Alphabittle Blossomforth, have not been known to have been referred to by her in animated media as Pippsqueaks, nor have adult viewers in Maretime Bay been.
Issue #11 of the comics (including official preview pages) had Pipp publish a post on what appears to be Ponygram, towards an otherwise unspecified Pippsqueaks group. The post had got 574 likes in-universe, indicating a large audience in a world with a total pony population estimated at less than 10,000.
Although the Maretime Bay Pippsqueaks bear similarity to the fillies shown at the end of A New Generation, they are not the same characters.
As for the latter, they didn't disappear after the movie and were shown helping Pipp and Zipp in Portrait of a Princess.
The synopsis of issue #17 of the comics mentioned Zippsqueaks who are fans of Zipp Storm instead of Pipp, which is likely a part of the cutie mark switch plot of the issue.