I don't feel sorry that they had to be "parents". It was their choice - I mean if they didn't want to help raise Simba, they could have seen to it that he got his own place to live within the jungle (it certainly seemed like a safe enough place). And the three got to have tons of fun together, and stick together throughout Simba's reign. I'd say there's far more win than lose.
And I'm not sure whether raising Simba was all that stressful for Timon and Pumbaa, considering 1.5 has so many continuity flubs with the first film, it's hard to be sure how canon it is.