The orcs insult and disparage elves but they use terms that seem too human -- is something being filth that big a deal to an orc? So are we getting only an imprecise translation of what orcs say often, perhaps only conveying sentiment without the real meaning? I think other species similarly refer to humans/elves/hobbits in this sort of way, IIRC trolls who also simply would not describe someone or something as filthy -- they would not notice. I could see using words like "weak" or "cowardly" or even insulting their intelligence but not hygiene of cleanliness of their clothing.
In the appendices, Tolkien actually addresses this himself:
“But Orcs and Trolls spoke as they would, without love of words or things; and their language was actually more degraded and filthy than I have shown it. I do not suppose that any will wish for a closer rendering, though models are easy to find. Much the same sort of talk can still be heard among the orc-minded; dreary and repetitive with hatred and contempt, too long removed from good to retain even verbal vigour, save in the ears of those to whom only the squalid sounds strong.”
Emphasis mine. Tolkien explicitly admits that the orcs' speech has passed through a translation filter, and he argues that it is done as he supposed no one had need for a more accurate translation.
Therefore, the real meaning of the word which is translated to "filth(y)" may simply be referring to something that to the orcs is considered filthy - even if that is not literal filth in the way that a human would label it.
It may just be translated by tone
One of our only examples of actual black speech is Grishnákh's curse "Uglúk u bagronk sha pushdug Saruman-glob búbhosh skai"
He stooped over Pippin, bringing his yellow fangs close to his face. He had a black knife with a long jagged blade in his hand. ‘Lie quiet, or I’ll tickle you with this,’ he hissed. ‘Don’t draw attention to yourself, or I may forget my orders. Curse the Isengarders! Uglúk u bagronk sha pushdug Saruman-glob búbhosh skai’: he passed into a long angry speech in his own tongue that slowly died away into muttering and snarling.
The Lord of the Rings - Book III, Chapter 3 - "The Uruk-Hai"
This sentence is sort of unique in that Tolkien has provided three different translations for it:
Uglúk to the cesspool, sha! the dungfilth; the great Saruman-fool, skai!
Manuscript draft of Appendix F, published in 1996 in The Peoples of Middle-earth
Uglúk to the dung-pit with stinkinq Saruman-filth - pig-guts gah!
Typescript draft of Appendix E, published in 1992 in Vinyar Tengwar #26
Ugluk to torture (chamber) with stinking Saruman-filth. dung-heap. skai!
"Words, Phrases and Passages in various tongues in The Lord of the Rings", first fully published in 2007 in Parma Eldalamberon #17
Of note is that the phrase "Saruman-glob" is sometimes translated by Tolkien as "Saruman-fool" and sometimes as "Saruman-filth".
This can perhaps be interpreted as that it was some Orkish insult, which doesn't necessarily actually mean "filth" or "fool", but could be translated equally well to both of those based on the tone.
That said, there are a lot of places where Orcs (and other races) refer to other people or places as filth or filthy. Perhaps Orcs did find the word to be an insult, maybe due to borrowing from common speech, or maybe just they thought it was insulting when applied to a person.
For example we see Orcs calling other Orcs filthy as insult.
Then you must go. I must stay here anyway. But I’m hurt. The Black Pits take that filthy rebel Gorbag!’ Shagrat’s voice trailed off into a string of foul names and curses.
The Lord of the Rings - Book VI, Chapter 1 - "The Tower of Cirith Ungol"
Gollum and the Orcs seemed to perceive the lembas bread and other Elvish items as tasting terrible, full of a smell they didn't like, and outright poisonous or painful - e.g. Gollum with the Elvish rope. Everything the Elves touched seemed to have an aura of ancient light, perhaps of Valinor itself – and while a hobbit or Man might experience this as heavenly and bracing, creatures who were exposed to Melkor or Sauron for a long time had a strong negative reaction to it, an allergy in a sense. So they really did experience Elves and perhaps other races as having a strong malodorous scent, and/or a piercing light that caused migraine headaches (speculating). With that in mind, their descriptions of the Elves & Co. as "filthy" or "stinking" were accurate, from their own experience.