If we're still talking about it two decades later (which we are), and people are still saying it when they see or want Taco Bell (which they are), then it more than paid off in the long run. It's kind of like politics in that it's a long-term strategy. One party screws up the economy, and the next administration gets blamed. Or one party fixes the economy, and the next one takes credit for it. That little Chihuahua was a genius campaign and Taco Bell is still benefiting.
OTOH, I actually wrote a whole novel featuring a talking Chihuahua, and it never occurred to me to have him crave Taco Bell. I should have put a reference to that in there, lol. (The book is called CHROO, btw, in case you're interested in such things.) :-)