What is a good player and what is a good GM ?
Yeah, I know, I hate when somebody answer my question by a question.
A good player could be seen as somebody who "solve" the scenario or as somebody who "stays" in character. I believe the 2nd one is the most entertaining, but if the story cannot progress because of the antics of one character...
Then there are all shades of gray that longtime players would learn when to stay in character and when to "bend" a bit there character for the greater good of the storytelling.
A good GM is somebody who is able to come with an interesting story (creating a plot which is not obvious, contains a few surprises and good NPC), can ensure a smooth game without downtime, can balance a fight (giving the feeling that the PCs risks there live without outright killing them), knows well the rules to not loose too much time looking in a book, immerse (how many times Tom shouted "Immerse me Ross!") the players... In this case, getting in character is only a fraction of the skills required to be a good GM.
In my case, my players regard me as a very good GM, but I am considering myself only as an average player because I do not stay always in character and tend sometime to let my knowledge override the character knowledge.
Is it possible to be good GM if you are a lousy player ? I guess it depends what makes the player a lousy player and how quickly he can learn. I believe that a GM with a good story, well organised, but not able to be good comedian will still be a good GM. Sure, his NPC will all have the same voice and tone, but a good story can make up for it. However, a lousy story told by a good comedian might be fun once, but afterwards, it will become pretty boring.
It is much easier to be a good player than good GM.