A lot of things can cause mismolts. Low/high humidity, low/high heat, stress, bad location, general wrong husbandry which leads to weak mantises. Depending on how finicky the species is even the lack of correct angled branches will cause deformities. So without knowing your setup and what type of mismolt it's very hard to tell what's going on, but if mismolts are frequent then there's definitely something going wrong. Usually I get about a 5-20% mismolt rate depending on the species during the first molt, after that mismolts become a lot rarer.