I lost a lot of weight just by portion control and what I ate. Cut out all the bad good things like beer and excessively fatty foods, sugary stuff, cook all my own meals without adding fats and switch to a more vegetarian diet. Didn't do any cycling and small amount of walking.
But, a year off the bike was still a year off the bike. I was thin, but I wasn't fit and it took a several months for my leg muscles to build up to a point where I can ride 100km in a day and pretty much ride that again the next day, but that's just time on the bike.
You'll still need to do weights or some kind of resistance training or just cycling will mean you won't have much arm strength (as for road cycling it's not needed and as you cycle you don't lift heavy objects) and if you want definition in your stomach you're going to need to situps or another exorcise that strengthens those muscles. Mountain biking is better for the arms because you gotta use em a lot more.
ANY form of cardio is good for you. Cycling, at least road cycling for the most part gets you out of your living space which is good as it can improve your mood / give you time in a different location. Cycling helps focus the mind. You're pedaling, constantly working with and against the elements, gravity and your own fitness to try and keep the bike at a pace you want it to be at.
Cycling is low impact so apart from damaging your knees if your bike is the wrong size, or tearing a muscle because you pushed too hard it's a fairly safe activity for your body.
You could achieve similar results by going hiking especially if your doing a lot of hills where you really work your cardio system.
You can even lose weight by having a lot of sex if you really get into it at least
Tip - Cycle because you want to and you enjoy it. Don't cycle because you think it's going to be a magic pill to transform you. If you enjoy doing something you're more likely to keep doing it.