(Computer science/Machine Learning) Is it better to take lots of math classes or AI/ML courses the first year of a PhD? I just finished up my undergrad and am starting a PhD in comp vision/machine learning this fall. I don't have as much math maturity as I'd like.

Do you recommend taking math classes early on or classes such as computer vision/nlp/robotics early on to get a better idea for my research focus?
Based on vague phrases such as "[not] as much math maturity as I'd like", you may get a spectrum of advice on this site. You should ask people who know you and your background and an adviser in the PhD program. 

Presumably, if your background (in math or other subject matter) is lacking, your new adviser will have noticed and will have recommendations how to remedy that.

Issues to consider: 

(1) There are probably some CS and ML courses you will need
to take at the beginning in order to be on track to finish your degree during the time the department is prepared to support you. My guess is that these
will not have heavy math requirements.

(2) You will not know everything you need to know even when you finish your PhD, much less at the start.
Learning much of what you need to know is a lifelong adventure.

(3) Your primary objective should be to find an interesting and do-able
thesis topic as soon as possible, to finish a high quality thesis on
schedule, and to get your degree. 

(4) A thesis requires original research. If you know exactly what you're
doing, how long it will take, or how much support you will need, then
the project is not research. So you need to anticipate uncertainty and
delay. Accordingly, you should not build any avoidable delays into your program.

(5) All other things being equal (which they never are), you might
consider looking for a thesis adviser who is in the middle of his or her
career, and heavily engaged in pathrbeadking work. Assistant professors may be so frantic trying to do their own
research to get tenure that they have little time to worry about yours. Some senior
professors are not entirely up to date, and may suggest topics that would
have been more appropriate five or ten years ago. A really promising
adviser may have great advice what courses to take early in the PhD program

(6) Most people who get into PhD programs are from the top of their
undergraduate classes. They cannot all be at the top of their PhD class.
There may even be some areas in which you are near the bottom of your
PhD class and you can hope for some in which you are near the top. You
may have as much to learn from some of your classmates as from some
of your professors. 

(7) Soon there may be other Answers disagreeing with everything I have
said. Maybe they will be right. But these are some issues worth considering.