Nej, Fett (enligt stavningen), är inte brandfarlig.
Det finns redan en stavning som gör det här:
Incendiary Slime
(Complete Mage, p. 108)
Level: Sorcerer 2, Wizard 2,
...
A slick, greasy liquid seeps from the targeted object, thoroughly coating it. This spell functions like grease (PHB 237), but the liquid is also highly flammable. If any fire damage is dealt within the area of the spell (or to the subject of the spell), the spell's area (or subject) bursts into flame. This effect deals 4d6 points of damage to anyone in the area (or holding the subject), but also ends the spell's duration. A successful Reflex save halves this damage. (emphasis mine)
De skulle inte ha skapat en nivå två stavning som specifikt kallar sig vara brandfarlig som skillnaden om nivån en stavning redan var brandfarlig.