Is it? Most people I know who have flags proudly displayed are left wing and their flags are usually one of: the Palestinian flag, the ukrainian flag, the LGBT rainbow flag, or the trans flag.
(I’m more likely to see the white rose of the House of York in “opposition” to the flag shaggers than a rainbow or anything else, in my neck of the woods, but there’s only a few of these flying)
I do like the wider interpretation though, that any ideology can blind you.
Personally I kind of thought of Russia which is about the only lot marching off to war with Russian and Z flags all over.
The St George lot mostly just moan about immigrants.
If they do, what do they stand for, and what would someone hanging one, versus the other, be communicating?
The sculpture's message isn't "flags are bad" - it's using a flag as a metaphor for nationalism/blind patriotism (based on the rest of the statue, the location chosen, what it's a response to, and Banksy's other works).