Assalamu'alaikum w.w.

The principle here is whether a t-shirt (short sleeves) contradicts the injunction of wearing respectable clothing in Salaat. The Fuqahaa mention it is makrooh-e-Tahrimi to perform Salaat in clothing which a person will not normally wear in a dignified gathering for example, one would not normally wear pyjamas in front of people hence Makruhi-e-Tahrimi to wear these clothes in Salaah.

With regards to short sleeves, those who wear them in and out of salaat will be considered Makrooh-e-Tanzihi, ( better to avoid).

Allah (SWT) Knows Best.
(Mufti) Abdullah Patel