Did Martin Luther take exception to the gospels in any way?

Yes and no. He preferred the gospel of John over the synoptic gospels, under his somewhat unusual view of revelational clarity within the canonical books. i.e. some books are more clear and powerful then others. For example, many would agree that Romans is more useful than proverbs in terms of communicating the message of salvation, but Luther was much more adamant about this kind of idea.

Luther was somewhat critical of the canon (in the modern biblical sense) partly because he distrusted the apocryphal additions made by the Catholic church and partly because of his suspicion of any book in the Bible that seemed in any sense to support what he understood as the Catholic teaching of works based righteousness. Therefore although Luther included James, Hebrews, Jude, and Revelation in his Bible (he even included the non canonical apocryphal books) he placed his own views of them in his ‘introductions’. In his introductions we find most of his canonical views.

Luther did not seem to have any objections to Esther, only to some portions not found in the original:

Six additions to the book of Esther, comprising 107 verses not in the Hebrew text, were inserted into the text in the Greek Version, added on at the end in the Vulgate. Oesterley, An Introduction to the Books of the Apocrypha, p. 183. The ancient Greek and Latin versions of the book of Daniel contain three principal additions not found in the original Hebrew and Aramaic text; these are included among the Apocrypha as separate “books”: The Prayer of Azariah and the Song of the Three Young Men, Susanna, and Bel and the Dragon. Metzger, An Introduction to the Apocrypha (New York: Oxford, 1957), p. 99 (uther, M. (1999). Luther's works, vol. 35: Word and Sacrament I (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.). Philadelphia: Fortress Press.)

Regarding Luther’s preference of books that clearly outlined the gospel over and above those that were less clear in terms of justification, Luther thought of the clearer ones as ‘noblest books’. He did not imply that the other writings were not canonical juts that they were less massive in their depth of revelation into what was ‘core’.

[Which are the true and noblest books of the New Testament] [From all this you can now judge all the books and decide among them which are the best. John’s Gospel and St. Paul’s epistles, especially that to the Romans, and St. Peter’s first epistle are the true kernel and marrow of all the books. They ought properly to be the foremost books, and it would be advisable for every Christian to read them first and most, and by daily reading to make them as much his own as his daily bread. For in them you do not find many works and miracles of Christ described, but you do find depicted in masterly fashion how faith in Christ overcomes sin, death, and hell, and gives life, righteousness, and salvation. This is the real nature of the gospel, as you have heard.

If I had to do without one or the other—either the works or the preaching of Christ—I would rather do without the works than without his preaching. For the works do not help me, but his words give life, as he himself says [John 6:63]. Now John writes very little about the works of Christ, but very much about his preaching, while the other evangelists write much about his works and little about his preaching. Therefore John’s Gospel is the one, fine, true, and chief gospel, and is far, far to be preferred over the other three and placed high above them. So, too, the epistles of St. Paul and St. Peter far surpass the other three gospels, Matthew, Mark, and Luke.

In a word St. John’s Gospel and his first epistle, St. Paul’s epistles, especially Romans, Galatians, and Ephesians, and St. Peter’s first epistle are the books that show you Christ and teach you all that is necessary and salvatory for you to know, even if you were never to see or hear any other book or doctrine. Therefore St. James’ epistle is really an epistle of straw, compared to these others, for it has nothing of the nature of the gospel about it. But more of this in the other prefaces.]( Luther, M. (1999). Vol. 35: Luther's works, vol. 35: Word and Sacrament I (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.) (361–362). Philadelphia: Fortress Press.)

Thus we see that although the Gospels of Matthew, Mark and Luke were scripture to Luther, they were not as clear as some other books, like the Gospel of John.

Regarding a book like Revelation, Luther has an odd freedom (and should I add confusion). One on hand he seems so unrestrained to be so bold as to deny its canonicity, partly because he was critical of anything not clear to his own understanding amidst the corruptions at the church at the time and partly due to its slow recognition into the canon within church history. On the other hand, one has to wonder why was he confused about its clarity of the gospel? To confuse the issue more he later changed his view and accepted it as canonical! (For the preface of 1522 his opinion was it was not canonical, volume 35 pp. 398–399 then from the preface from 1530 p. 411 he accepted that it was).

Also Luther was not so dogmatic about ‘his opinion’ as it was changing amidst the dramatic theological revolutions in which he seems to be at a center place. He was very ‘free’ in having a non-dogmatic opinion and willingness to express it without fear of being put to death. In his earlier introduction of Revelation he shows his 'opinion' approach while granting others free opinion as well:

About this book of the Revelation of John, I leave everyone free to hold his own opinions. I would not have anyone bound to my opinion or judgment. I say what I feel.( Luther, M. (1999). Vol. 35: Luther's works, vol. 35: Word and Sacrament I (J. J. Pelikan, H. C. Oswald & H. T. Lehmann, Ed.) (398). Philadelphia: Fortress Press. )

This example really shows what Luther was all about. He was a leader whose vision was changing and adapting as he went along transforming Christianity into a more Christ centered establishment that answers the question 'What about me?' He was not the type of theologian that writes a few things, never changing his mind like Calvin's institutes built upon the groundwork of Luther. Rather as a true pioneer and leader of a populace he, recognizing many errors at the time and while leading a new understanding of what the Bible was really saying, steered away from anything that would get in the way of that vision, even distrusting (temporarily) a few Epistles he did not have a confident understanding about. Most likely the final protestant canon that is used today would have been Luther’s view if he lived a bit longer.