Short answer: Most Jains do not eat mushrooms.

Why:

• Ahimsa (non-violence) is central. Mushrooms are fungi and may harbor tiny life forms; eating them is seen by many Jains as causing more harm to living beings than other vegetables. This leads many to refrain from mushrooms. ( jainknowledge.com)
• There is no single universal rule for all Jains. Practices can vary by family or community, but the common tendency in both Digambar and Śvetāmbara traditions is to avoid mushrooms. ( jainknowledge.com)

Notes on tradition:

• Digambar and Śvetāmbara followers both largely align in avoiding mushrooms, though individual choices may differ in less strict households. The overarching aim is minimizing harm to all living beings. ( jainknowledge.com)

If you’d like, I can point to concise Jain explanations from Jain Knowledge that discuss this topic in simple terms.