It's usually fine though it's possible and there's a lot of variables so it is unpredictable. Creams, stretches, whatever won't help, it's just about how the skin happens to re-arrange itself relative to the design, you can't control it.
Forearms are lower risk since they won't change size so much but the quicker you loose weight the higher the risk and that is a lot of weight. If it's not symmetrical or geometric or with lots of detail then you probably won't notice any distortion all that much.
Though the obvious question is, why not phone up the artist and ask if postpone for a few months? You'll have the tattoo forever so in the grand scheme of that a few months isn't long. Even if you wait til you loose half the weight you'll significantly reduce the risk. Or keep it as an incentive, tell yourself no tattoo til you loose the weight.