I got 80 percentile in jee mains and 56% in Gujarat board belong to ST category so can I get CS branch in MBM jodhpur

Hello.
Well minimum eligibility percentage in your 10+2 must be at least 40% for MBM college , as you are having more than it so from this side you are eligible but if your mentioned percentile is of JEE mains January session of exam so I would like to inform you that MBM is one of the prestigious college and having good ranking so for that if it comes to General category so you may consider your good chances if you have percentile more than 98, with this your rank may lie under the cutoff but there may be variations because predicting directly from percentile will give ambulatory results as your percentile is prepared on the basis of the number of candidates who get marks less than and equal to your marks but in order to convert it into common rank we can use below relationship for approximate idea and here total number of candidates appeared for the exam vary for every session of exam so your rank from percentile would also vary .
Approximate Rank= (100- your score)* total number of candidates appeared for any particular session of exam/100.
Remember this formula is specifically for JEE mains percentile to approximate common rank conversation , so here in January session of exam number of candidates appeared was 8,69,010 so your approximate common rank with 80 percentile would be near about 173,802 now with this rank your chances of getting in your mentioned college even with ST category are less.
Now if you want to predict more colleges with your mentioned percentile score then you can go through the mentioned link and can use our college predictor tool:

Now you have another chance to improve your JEE mains score then you should appear for the next session of JEE mains exam ,here NTA has reopened application window from 19/may/2020 to 24/may /2020.
Best of luck.