Yes, on est les misérables is perfectly grammatical in spoken French where on has almost completely superseded nous as a subject.
Speaking about les misérables, Victor Hugo, Les Misérables author who isn't especially known to use "improper French", "low French" or for being a football fan (Sorry @CédricVanRompay, I couldn't resist ;-) ) wrote these verses in his Alsace et Lorraine poem 1872, Toute la Lyre:
...
Se ruant sur l'auguste et sombre genre humain.
On est les chefs de l'ombre et l'on a dans la main
Les rênes des chevaux du sépulcre, on excite
...
On gonfle le flot noir des légions sinistres
On est les dieux ayant les démons pour ministres ;
However, in your sentence les misérables looks odd.
Perhaps did you want to say:
on est misérables
on est des miséreux
on est dans la misère
?
In any case, a famous phrase using the on est les pattern is On est les champions.
A written, formal document will likely use nous sommes les ... though.