The mistake here is thinking that the Borg are obsessed with tech. They're also known to have assimilated species that are (to use their own terminology) "primitive" when a species distinguishes itself physically or because some aspect of their culture has intrigued them.
SEVEN OF NINE: It began with Species Two-Six-Two. They were primitive, but their oral history referred to a powerful substance
which could "burn the sky." The Borg were intrigued... which led them
to Species Two-Six-Three. They, too, were primitive.. and believed it
was a drop of blood from their Creator.
Voy: Omega Directive
SEVEN: You're wrong. They are here. Vulcan. Species three two five nine. Your enlarged neocortex produces superior analytical
abilities. Your distinctiveness will be added to our own
Voy: The Raven
It appears that it's only when a species combines the two facets of being both primitive technologically and physically unimpressive that the Borg will ignore them.
SEVEN: Their [The Kazon's] biological and technological distinctiveness was unremarkable. They were unworthy of assimilation.
Voy: Mortal Coil
As to the question of whether the Borg would assimilate the Mugato, the answer is that they'd almost certainly assimilate (or possibly just scan) one of them to create an assimilation profile. If they were found to be useful (perhaps if their minds could be augmented or their physical strength harnessed in some way) they'd assimilate the entire population. More likely they'd leave the place alone until the Mugato had evolved to the point of being interesting.