If you are claiming you lost income as a result of the injuries you received in the incident that gave rise to the lawsuit, the defendant has a right to determine if you lost the income you say you lost. One way of determining that is looking at your income tax records.
However, the fact that tax records may reveal whether you lost income that does not mean that your entire tax return can be seen by the defendant. Tax returns contain lots of information other than income, and that information has nothing to do with whether you lost income.
An experienced personal injury lawyer will determine whether it is appropriate to release your entire tax return to the defendant or whether only some portions of it (such as your W-2, which is a statement of income from your employer) is appropriate.