Mongo foot is when the skater takes their front foot off and pushes the board with the front foot while keeping their rear foot on the rear portion of the board. This is one of the few techniques not invented by Rodney Mullen.
This method of pushing is sometimes harshly considered inferior by many skaters, as it reduce efficiency and control. It also increases the setup time for tricks because the rear foot needs to be re-positioned after every push.
If you are just getting started and you started off pushing mongo, it is recommended that you switch to pushing normally with your rear foot. This will go a long way toward making you a better skater. But, then again, if you are happy pushing mongo, that is perfectly fine too. If you are having fun, you are doing it right. Don't let anyone tell you how to express yourself. Just have fun! No one should be judging you, unless you are participating in a contest, even then, it may not matter. There are some highly respected skaters out there, like Bob Loftin, pushing some bitchin' mongo.