As of my understanding, unless you configured for certificate authentication, certificate is not used for user authentication. the wild card certificate you are using will be presented by ASA to user while you access ssl portal ( like normal https access of web sites) or when you access through the client software. Client will check the authenticity of certificate(whether it is signed by trusted CA/certificate is expired or not/common name is matching?..etc). so if your certificate is expired, error will be shown in user machine. user have to accept it then only they will be directed to the portal.