Diet is more important to weight loss than your intended exercise routine.
But exercise is still a good thing.
The cheapest Fat Bikes come w/o front brake, avoid them.
Fat Bikes do great on soft surfaces, but tend to roll and steer heavily on roads.
Unless you ever plan on riding on snow/mud/sand I suggest you look for a hybrid/regular width bike instead. If you get into riding more seriously, there’s ”always” a reason to keep a utility/bad-weather/spare bike around.