You need to modify the solver settings under configuration parameters. To access configuration parameters,
Go to the home tab under modelling in simulink -> model settings and click on model settings, change your solver type to variable step and solver to auto (Automatic solver selection) -> click okay and rerun the model,
hopefully the issue will be fixed and you should be able to see smooth sine wave. If you are just getting started with Simulink then the following resource will help you.
I hope this answers your query
Thank you.
