Having more body fat can mess with your gains regardless of which compound you take. Deca doesn't convert to estrogen as much as testosterone does but higher body fat can still increase aromatase activity which might ramp up those estrogen levels. Too much estrogen isn't what you're aiming for when you're building muscle it causes water retention and fat gain.
Now deca has this thing where it messes with your hormones in other ways too like through progestogenic effects. This can bring on sides like gyno more easily, especially if your body fat is higher. Plus, when you've got more body fat, your insulin sensitivity might drop. Not ideal cause insulin's key for muscle growth. It's always smart to trim down body fat before starting a Deca cycle or any cycle really. This way you're keeping the gains lean and clean without any extra fluff.