Matt was my first Mark, and has been my only Mark, for the 5 times I have seen the show so I'll admit I am a bit biased because I have nothing to compare him to.
However, I think he does a wonderful job! I think the boy has a good understanding of when it is time to have fun and when it is time to get down to business. I feel like the ACT I Mark should be a little playful and relaxed. Matt's Mark I feel provides entertainment and laughs to the audience, which IS needed. Much of the play is serious and audiences need to enjoy a good chuckle. Everytime, for example, ACT I Mark pulls out a little dance move in Tango, Maureen, I feel as if the humor is well placed and works.
ACT II Mark I think should be more subdued and personally I have never seen Matt goof off during ACT II. In fact, he always kinda breaks my heart a little bit during "Halloween" and "Goodby Love."
Bottomline, I think he plays Mark very realistic. Yes, Mark is suppose to detach and be quiet and introspective, and whatever else people are making arguements for, and he does. But to balance out the character properly Mark should be the character to bring in a little lightheatedness, which Matt does.
Leave the brooding to Roger.
"Take all of your so-called problems, better put them in quotations.." - JM