WoW isn't very GPU heavy, the culprit is more likely a not-too-gaming-heavy i3 that is compounded by being the under powered "U" version (though don't get me wrong, with some good RAM a desktop i3 is fine for moderate gaming...speaking of, how much RAM do you have?) Granted, the integrated graphics don't help but that's not going to affect FPS a whole hell of a lot in WoW unless you've maxed everything out, and the major telling factor regarding CPU performance is the fact that the FPS drops heavily when more characters are on screen. Sadly there's probably not too much you can do.
Don't waste your money on a fan. Unless your CPU is thermal throttling due to intense heat, it's not going to do anything at all (and I'm actually not even sure if this occurs with the U models, though it almost certainly does with newer ones; older models might just power down). Cooling doesn't affect performance in that way and it isn't meant to. Cooling helps PCs "perform better" because the temperatures are lower, thus they can amp up the volts without overheating but it won't help you attain a higher FPS rate because a higher FPS rate requires more power, not reliability/sustainability. If you were running the game at max settings and it was running at 60FPS, yet you were getting temps of 80 C after not playing all that long, that's when a fan would really help out. I don't know too much about upgrading laptops that aren't meant to be upgraded (i.e. that aren't marketed as being upgradable or that aren't gaming laptops), but I'd bet on the option being pretty unlikely with an Inspirion.