Of American-English origin, the term pompom girl designates a female cheerleader who uses pompoms.

Here, the noun pompom denotes a large round cluster of brightly coloured streamers shaken or waved in pairs by female cheerleaders in support of a sports team, such as a college football team.

An early instance of pompom used in that sense appeared in the caption to the following photograph, published in the sports section of The Los Angeles Times (Los Angeles, California) of Thursday 28^th October 1920; the football teams of Pomona College (Claremont, California) and of the University of Southern California (Los Angeles, California)—i.e., respectively, the Sage Hens and the Trojans—were to compete at Claremont the following Saturday:

Here’s Luck, Ed—You’ll Need It.

                                                   Big Ed Covington of Pomona,
Who will lead his Sage Hen football aggregation out against U.S.C. Saturday in a final conference effort to heave the Trojans into the junk heap. Capt. Covington has a speedy eleven this year and the battle at Claremont ought to provide thrills enough to keep the pom-poms violently agitated.

I have found an early instance of pompom girl—which may correspond to the definition—in the Stevens Point Daily Journal (Stevens Point, Wisconsin) of Friday 28^th February 1913:

BASKETBALL TOMORROW NIGHT
The Principal Game Will Be Local Normals vs. Whitewater Normals.

[…]
When Whitewater plays here tomorrow night the locals will meet their first antagonist in the southern division. Therefore a splendid game is expected, and the Stevens Point team will make every effort to carry the heavy end of the score. It is regretted that the Normal orchestra cannot appear on Saturday night, but Miss Parkhurst’s pom-pom girls will again appear in a new and varied stunt.

The second-earliest occurrences of pompom girl that I have found date from November 1934 and are from The Sun-Democrat (Paducah, Kentucky):

1: Thursday 8^th November 1934:

Tilghman to Have Parade on Saturday

Saturday will be “School Children’s Day” at Keiler Field and the biggest parade in local football history is planned for the occasion.
[…]
Participating in the gigantic parade will be the Tilghman band, girls’ drill corps, boys’ cheering group, the Tilghman “pom pom” girls, students of Tilghman three junior high schools and children of the fifth and sixth grades of every elementary school in the city.

2: Monday 12^th November 1934:

HOMECOMING TO BE OBSERVED AT TILGHMAN HIGH
Alumni Invited to Assembly Program At School Friday Morning

The Hopkinsville-Tilghman football game at Keiler Field next Saturday will be the occasion for a homecoming celebration at the high school.
[…]
Another big parade is planned for next Saturday morning. It will start at Eleventh and Broadway where the Hopkinsville band, fans and team will come in on a special train. […]
Hopkinsville’s 60-piece band will march in the parade with Tilghman’s band, girls’ drill corps and pom pom girls.

3: Thursday 15^th November 1934:

Tilghman Homecoming Will Start Friday and End With Hoptown Contest Saturday
Blue Tornado Scrimmages Against Tricky Hopkinsville Plays

[…]
The Hopkinsville squad will arrive in Paducah Saturday morning on a special train with the school band and hundreds of fans who are attracted by Hopkinsville’s great success this fall.
The Tilghman band, girls’ drill corps, pom pom girls and decorated automobiles of students and alumni will meet the Hopkinsville special train at Eleventh and Broadway, and a parade will be staged in the downtown district.

4: Sunday 18^th November 1934—in the account of the football match between Hopkinsville’s and Tilghman’s teams:

Approximately 600 fans from Hopkinsville came to Paducah for the game. About 300 of them came on a special train. Tilghman students and Alumni met the special train, and a huge parade, led by the 65-piece Hopkinsville high school band, was staged. Tilghman’s band, girls’ drill corps and pom pom girls participated, in addition to students and alumni in decorated cars.

The next-earliest instance of the term pompom girl that I have found is also from The Sun-Democrat (Paducah, Kentucky); on Friday 29^th November 1935, Louise Livingston, the editor of that newspaper, wrote this in Sideline Sidelights:

Football!….Ah, what a game!…And each year becoming more and more a favorite of the women….such fact was very much in evidence at Keiler Field, packed to capacity Thursday afternoon…..[…]
[…]
The sweet smile of a young sub as he comes out….he has gone in for only a minute….the good-natured pats on the back given by opposing team members…..the pom-pom girls in the cheering section….[&c.].

These photograph and caption are from The Hammond Times (Hammond, Indiana) of Monday 15^th October 1945:

Yeah, Team! Yeah Team! Fight! Fight! Fight!

If the Pasadena, Cal., Junior College team is chalking up a good record on the gridiron this fall perhaps they are being inspired by this quartet of cheer leaders. The gals are known as the “Pom Pom Girls” and strut their stuff at all the college games.