Despite having to transport a significant amount of mass to Mars, would lead lined environment suits be viable, to provide cosmic radiation shielding for Martian colonists? Would carrying that extra weight help mitigate the health impacts of the low gravity?
Environment suits are likely only going to be worn for a small fraction of an astronaut's time on Mars. The benefit as a counter for the low gravity would be minimal, the extra weight would make those already-hazardous activities more tiring and make the suits more cumbersome to put on, and the extra layers would directly obstruct inspection and maintenance of the more important structural and airtight layers, and would likely mean a penalty to suit strength and durability.
It might provide some radiation protection, particularly secondary x-rays from charged particles striking the ground, but it would also cause more such radiation from charged particles striking the suit directly. Whether it provides a benefit depends on the precise composition of radiation at the ground. However, we know the total intensity of that radiation is relatively low (lower than experienced on the ISS), astronauts will spend most of their time "indoors", and even an EVA-suited astronaut outside is likely to spend most of their time in a vehicle or operating a piece of machinery which could itself incorporate far more shielding than an astronaut could wear.
In short, you're probably better off with more protection against wear or damage like punctures or cuts. Additional layers of polymer fiber will themselves be relatively good shielding against particle radiation, and suit damage seems likely to be a greater hazard.
Short answer: It depends
Longer answer: We don't know what health effects lower gravity has on the human body. So if a lead lined suit would help in that regard is therefore unknown.
Lead is a commonly used material for shielding because it allows us to get a lot of mass in a relatively small space. Lead lined aprons are used in radiology and operation theaters to protect personnel and patients from radiation. So it's proven to work. But if you ever had to wear that stuff you might also know that it's not comfortable and quite heavy. While the weight might not be a problem in Mar's lower gravity, inertia stays the same as on Earth. So moving that mass around will cost a lot of energy.
It would probably be better to just keep the exposure time as low as possible and spend your time in an underground habitat (maybe even under your water reserve, because water provides incredibly good shielding from radiation) where you're well protected.