First of all, I don't see how this is considered "homeopathic" because it is sold in 1X concentration. This means that the active ingredient is 10% concentrated and thus the drug works like any other herbal product.
Secondly, the plant itself is known for its medicinal properties as an antiseptic - and not as a skin irritant as homeopathy would need it. This is obviously not a "homeopathic" medicine but a herbal medicine.
More specifically to your question, there have been clinical studies, sponsored by Boiron but published on peer-reviewed journals attesting its efficacy:
Calendula is highly effective for the prevention of acute dermatitis of grade 2 or higher and should be proposed for patients undergoing postoperative irradiation for breast cancer.
Phase III Randomized Trial of Calendula Officinalis Compared With Trolamine for the Prevention of Acute Dermatitis During Irradiation for Breast Cancer,
P. Pommier et al., doi: 10.1200/JCO.2004.07.063, JCO April 15, 2004 vol. 22 no. 8 1447-1453
That there is plenty of other scientific literature not contesting its value as a folk remedy for skin irritation.
The toxicity of the component needs to be studied more according to a 2001 report, at least for large-scale cosmetic use:
[...] it is concluded that the available data are insufficient to support the safety of these ingredients in cosmetic formulations
To be clear, this should not be taken as a particularly warning sign as the component is relatively non-toxic in animal testing:
Acute toxicity studies in rats and mice indicate that the extract is relatively nontoxic. Animal tests showed at most minimal skin irritation, and no sensitization or phototoxicity. Minimal ocular irritation was seen with one formulation and no irritation with others. Six saponins isolated from C. officinalis flowers were not mutagenic in an Ames test, and a tea derived from C. officinalis was not genotoxic in Drosophila melanogaster. No carcinogenicity or reproductive and developmental toxicity data were available. Clinical testing of cosmetic formulations containing the extract elicited little irritation or sensitization.
Final report on the safety assessment of Calendula officinalis extract and Calendula officinalis, Int J Toxicol. 2001;20 Suppl 2:13-20.