Well, in Iskalla Trollblodet(=The Ice-cold trollblood) the trolls are singing an epic about the Hall of RivFader(their heaven) which is basically Valhalla. Lot's of drinking and fighting and where the drinkin horns are never empty. The songs theme changes drastically when the cheerier parts come along.
In RivFader(TearApartFather) the chorus goes something like this: "In deep deep forest, runs a wolf. In deep deep forest burns a flame. RivFader will come back to eat the flesh of christians when the white serpent is seen on the northern skies"
In Bastuvisan two priests (Aamund of Kettil) go to sauna together, but the trolls attack and give them a beating, effectively blowing up the whole building in the process.
There's one song about hypnotic mushrooms, one song about siege equipment to crush humans, one song about pillaging boats and one song about a legendary giant mushroom.
Edit: Finntroll is supposed to be cheese. With such song themes, what else could it be. The other band of the group resbonsible for the songs(Moonsorrow) is the more serious one.