Sexual dimorphism may affect the reproductive success of the species. However, it is not a simple relationship where no dimorphism leads to lower success and higher dimorphism leads to higher success.
There are various theories explaining dimorphism. For example, it is theorised that in monogamous species females tend to be larger: Larger females have a reproductive advantage because larger bodies can store more resources for producing offspring, hence, increased fertility. Larger size in males is shown to be related to male-male competition, intrasexual combat or territoriality (for example, here).
Please note that sexual dimorphism is a very complex phenomenon. And its presence or absence are not determined by 1-2 simple criteria, especially in intelligent species. For example, patterns of sexual dimorphism in human faces vary across cultures and are influenced by a huge number of different factors: Aesthetical preferences, evolutionary pressures, habitat, lifestyle, diet, etc.
Lack of sexual dimorphism may be adaptive and promote reproductive success. For example, a recent study of Malagasy mammals suggests that unusual climatic unpredictability on Madagascar have ultimately reduced SSD (sexual size dimorphism) in lemurs after dispersing to Madagascar. It is speculated that resource unpredictability resulted in an increase in the body size of females due to adaptations that either maximize energy intake or minimize energy expenditure.
As these examples show, it is not possible to state with 100% certainty that the orcs population or reproduction will be negatively impacted due to their lack of sexual dimorphism. It may or may not be affected. You can choose whatever fits your narrative better.
The described culture, though, does not seem to favour sexual size dimorphism since you are suggesting mate-mate combat, not male-male combat. In other words, female orcs should be strong enough to fight male orcs.
Some other factors that could reduce sex dimorphism are an egalitarian society, co-parenting (or raising children as a community), monogamy, seasonality and high variance in resource availability, and cultural preferences for same-size mates.
No, lack of sexual dimorphism does not necessarily result in a negative impact on the reproduction of orcs.