Electronic configuration of Thallium is $[ Xe ] \,4 f ^{14} \,5 d ^{10}\, 6 s ^2 \,6 p ^1$.

$d$ and $f$ electrons of inner shells provide poor shielding, thus the attraction of the nucleus on the $s$ electrons of the outermost shell increases. This is called the inert pair effect.

Whereas, $s$ electrons provide strong shielding to $p$ electrons, thus making it easy for the $p$ electrons to be released.

Thus, although Thallium shows $+3$ and $+1$ oxidation states, $+1$ is more common.