He has not been awful for Bayern - not if you look right into his game. He had a shooting crisis from August to about March in which he just did not get much into the goal - a lot of forwards etc. have this sometimes - but he is second in assists in the league. Usually he has about as much goals as assists - but this year he just had 10 goals and 20 assists in all club competitions - that is still a number most players never get - and playing between Robben and Ribery certainly does not help his numbers as they work on own account... In the national team he had 3 goals and 9 assists this season in 613 minutes playing time - against decent opponents, no San Marino, VAE or Liechtensteins. He actually had as much scorerpoints (assists and goals) as Ribery for Bayern this season (who is told to have had a great season) - if you only take the results until a 2-goal-lead and forget all about the showoffs / and he is Bayern's best in scorerpoints in away matches, where the team really had problems this year.
But - he is nobody you can praise for the usual skills - he is only an average player in this. And as a lot people just watch the ball action they do not really see what gem he really is - there is no similar player to him, nobody that is such good without the ball. He can use zones better that anybody else, can fool opponent players with not important runs or move his fellow players can take advantage of - or opens spaces for others to move in - or holes to shoot through - he is permanently in move and is just always half a second faster than anybody else in thinking - and he can make a system like this very unpredictable. But - he needs a system that can really take advantage of it - Özil and Klose and the German team with its young players can - there he is very important. At Bayern you have have this great stars Robben and Ribery and a static forward in Gomez - they are great but want to do their own thing - so Müller is only in a support role. He can drive you nuts with his own ball actions - they go from genius to donkey - but he always offers a lot more than this. He was somehow in all of the goals for Bayern this year. Gomez does not score when he is not somewhere near. And Robben does his great matches next to him whereas he never shines with Kroos as CAM.
But - apart from all of this - the best about him probably is his mentality - and he has balls...
He was very careful in the press not to critisize things to open - but he cannot be at all satisfied with everything in the club this year. He is 22 - he still wants to get better. With Heynckes as coach he will not.