Hi all, I am an incoming engineering undecided. While UMD engineering ranking is good overall (50 globally by some sources ), chemical engineering specifically has a very low ranking ( around 150?). Why is UMD chemical engineering so average compare to other engineering majors ? Thank you in advance . 
When you say you are incoming engineering undecided, are you asking as an applicant or as an enrolled freshman? If the latter, did you sign up for VIRTUS? or ENES181? Both of these will give you an opportunity to ask about the chemical engineering program and address that question with faculty…
FWIW, I would not chose a specific discipline within engineering based on rank, but rather do so based on personal interest. You will be more successful doing something you enjoy rather than something that you think will be more impressive based on various rankings that will likely change over the years. Hell, when I went to Maryland (which was technically the last century…ugh!), the school was known as a party/safety school for most students. Overall, the school was not ranked nearly as high as it is now (even though engineering was still well-respected --even back then).
In general, I think an engineering degree from UMD is highly respected regardless of discipline. You will be hired for jobs based on personal record of academic success and personal presentation at interviews. Everyone knows that admission to Maryland engineering is VERY competitive, so don’t let an outside ranking dissuade you from pursuing your interest.
Discipline rankings are about graduate school and citations of research papers. These are irrelevant to undergraduate programs.