Thallous chloride is more stable than thallic chloride because of

TlCl -Thallous chloride →Tl⁺¹

TlCl₃-Thallic chloride →Tl⁺³

For "Tl" +1 oxidation state is more stable,due to inert pair effect.

NOTE: For III_A group higher oxidation state is due to involvement of ns and np electrons.

                IIIA group ns²  np¹         +3 oxidation state

Lower oxidation state is due to involvement of only valency "np" electrons.

              III_A group ns² np¹      +1 oxidation state

Higher oxidation state stability decreases from top to bottom.

Lower oxidation state stability increases from top to bottom.

Stability order of +3 oxidation state: B⁺³ >Al⁺³ > Ga⁺³  > In⁺³  >Tl⁺³

Stability order of +1 oxidation state: B⁺ > Al⁺ >Ga⁺¹  > In⁺¹ >Tl⁺¹

Therefore, Tl⁺³ is acting as strong oxidising agent.

Due to poor shielding effect by the innershell f- and d-orbitals electrons ,the valency s-orbial electrons are strongly held by the nucleus,therefore valency s-orbital electron pair does not involve in bonding [inert pair] and hence only one p-orbital electron involves in bonding and shows mainly +1 oxidation state.  Hence +1 oxidation state is stable for Tl is due to inert pair effect.