I'm not sure a band has to be a commercial failure to be mediocre. Mediocrity might actually just mean that they are so inoffensive that nobody can take exception to them and so the CDs just get bought and bought, sheeplike. From supermarkets and petrol stations. By people with german shepherd dogs called Prince. OK, I'm turning into Half Man Half Biscuit now! Annie Lennox!
Sure yeah, a band doesn't have to be a complete failure to be mediocre, but Simply Red and the huckster have acheived what can be described as a significant level of success in the music industry. C'mon they were nominated for (cough) Brit Awards!