Hi & welcome,
In addition to other's good info, yes, 'sweet feed' is the junk food of the equine market, generally. It is not good for any horse, just that some types/breeds, like Morgans, heavies, ponies, etc are more likely to develop serious issues from unhealthy diet than others.
Horses are built for 'low grade'(compared to 'improved' cattle fattening pastures we've got used to) roughage & are not made to cope well with high sugar/starch, grainy feeds or infrequent feeding. Generally a healthy horse should do well enough on grass/grass hay and appropriate nutritional supplementation for it's situation.
If extra weight is needed, a little lucerne, copra, rice bran & other 'low GI' options are generally better than grain or junk food. That your horse retained her weight well enough when feeding the foal & only lost a little when she weaned him makes me think she's a very 'easy keeper' & it was just the stress caused weightloss. Therefore I wouldn't be in any kind of rush to pump a 'weight builder' into her, or for that matter, add to what she's getting now. You may find that free access to grass hay is too much for her. A small holed net over it can take care of that, by slowing intake.
I would however, get her on a good 'ration balancer' or other nutritional supps appropriate for her situation, including extra magnesium(look up magnesium for horses), as the foal will have pulled the nutrients out of her.
Also being just 4yo with an already weaned foal, I'd want to get a good vet chiro to check her & her foal out ASAP, as immature mothers can have/cause in the foal extra body issues, which can more likely/successfully be treated while they're still immature.