I always think it's silly when players do this as they needlessly get a yellow which could cost them a ban at some point. I understand when Ronaldo does it because he's a vain twat and wants to show off his abs, but I kind of expected better for Salah. Imagine if he picks up another yellow and gets suspended which could cost them some point. Keep your shirt on and celebrate as much as you want.
More than being a show-off, Ronaldo always try to show superiority complex over Messi. He is always running a race against Messi, don't know if Messi even acknowledges his efforts or not. Messi removed his shirt in the El Classico prior to that one. But Messi was playing at Santiago Bernabeu and match was at 2-2 and he scored in extra time which was his 500th goal. This all justify why he removed his shirt. But the Ronaldo's act was purely envious based and he was rightly punished as he received next yellow card just 2 minutes later and was sent off.
I won't compare Salah's act with Ronaldo's as he removed shirt because of the joy of scoring against Man Utd but that may send negative message to people. It may have looked like Salah was excessively desperate to score. I could have imagined something like that from Lingard who is not able to score even a single goal this season but Salah already have 11 in his basket. Jokes apart, it is kinda disgraceful for the opponent.