If "mushy" refers to texture:
* Firm
* Crispy
* Solid
* Crunchy
If "mushy" refers to sentiment:
* Strong
* Tough
* Hard-hearted
* Unfeeling
If "mushy" refers to a weak or ineffective argument:
* Strong
* Cohesive
* Logical
* Convincing
It's important to consider the context of the sentence to choose the most appropriate antonym.