As always, it depends on what metric or take you use to determine it, as there isn't just one way of judging these things.
But if we're talking about biggest girl group song in terms of popularity in SK, then yes, Red Flavor was the biggest girl group song in 2017.
Other songs like IU's 'Through The Night' and YJS' 'Like It' were bigger according to Gallup (resp. #1 and #2 2017 top song), but of the idol group songs, not just gg songs bu bg songs included, Red Flavor was biggest (#5 in 2017 Gallup ranking).
For those who're curious how the Gallup listing for 2017 looked like:
1. Yoon Jong Shin — “Like It” (8.8 percent)
2. IU — “Through the Night” (4.3 percent)
3. IU – “Autumn Morning” (4.2 percent)
4. Jang Yoon Jung – “Call the Soul” (3.3 percent)
5. Red Velvet – “Red Flavor” (2.9 percent)
6. BTS’s “DNA,” Jin Sung’s “In Andong Station” (2.7 percent)
8. TWICE’s “TT,” Wanna One’s “Pick Me” (2.5 percent)
10. PSY’s “New Face” (2 percent)