You need to take your own firmness preferences into account, but yes, generally speaking, soft or soft-medium mattresses are the best choice for side sleepers because they offer more cushioning around high impact pressure areas such as your shoulders, hips and knees.
This is vital when you sleep on your side for hours at a time, because you’ll be putting continuous pressure on those parts of your body. Without ample cushioning and pressure relief, you’ll experience pain which will wake you up from sleep.
However, you don't want to opt for a mattress that allows your hips to sink in too far, as this will mean your spine isn't correctly supported. This is also the reason why stomach / back sleeper mattresses need to be firmer. The good news is that lying on your side is, for most people, the healthiest position (and it's also a good position to sleep in when it's hot).
Memory foam mattresses are generally regarded as a good choice for side sleepers because they create a cradling, body-hug feel that also relieves all pressure on your body during side sleeping. You can normally find these in soft-medium and medium-firm, both of which would suit side sleepers.