Yes, on est les misérables is perfectly grammatical in spoken French where on has almost completely superseded nous as a subject.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 Celebs interviewerédricVanRompay, 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.

The use of on in place of nous is considered as informal [1],
but in certain structures in just sounds weird,
and “On est les […]” is one of them.
“On est les misérables” will not sound “incorrect” but just… “low French”.

@jiliagre mentions “On est les champions” which is indeed a thing,
but it comes from a very specific context, namely football fans,
which is not considered as a reference for proper language.
Actually the fact that it “sounds a bit weird” might have help it becomes popular.

However it is interesting to note that “On est + [adjective]” is not considered as improper.

[1] There are other uses of on which are very formal: “On voit clairement que …”, “On retiendra que…”