"Stellar" is a slang word for awesome/amazing/great. Less-than-stellar means the opposite: not stellar/not great. "It's the responsibility of doctors to help those who *don't feel great* to determine the underlying health issues." I think a more common use for the word would be: "The musician gave a stellar performance". (They gave an amazing performance). Where I'm from (southwest US) it's not a word that's used very often in everyday conversations.

The meaning is that it is less than satisfactory or pretty mediocre

It means not great, or not ideal.