why are muons called mu-mesons if they're leptons?
Because historically the word meson meant a particle whose mass was intermediate between the electron and the proton. From the Greek for "midway thing". This was long before Gell-Mann thought of quarks. Only as the standard model was being put together, about 20 years later, did the present meaning of meson arise.
On the topic of misnomers... Isospin was once called isotopic spin. A less misleading name would be isobaric spin. The tacit agreement seems to be to just shorten the word to isospin.
As someone else mentioned, the word "meson" comes from "middle one" because the first discovered had a mass intermediate between the masses of the electron on one hand and the masses of the proton and neutron on the other hand.
The link between "muons" and "mesons" is due to a confusion that lasted several years in the early 40's. Yukawa had predicted a particle that would carry the nuclear force between nucleons, a particle of a mass a little more than 100 MeV/c^2. Physicists soon detected a particle of about that mass in cosmic rays. It was then called the meson (well, mesotron first then the name meson was adopted). It had a mass of about 103 MeV/c^2. And then they found one more (about 135 MeV/c^2), and then more and more!!
It was later realized that the first meson was *not* the carrier of the strong force. It was a new lepton (prompting Rabi to ask "who ordered *that*!?!). That's what we now call the muon. The particles near 135 MeV/c^2 (there are 3 of them, with charges 0,e,-e) are the pions and these are the particles predicted by Yukawa.
So it's for historical reasons that you sometimes see the name "mu-meson", because of the confusion between the muon and the mesons.
Separate names with a comma.