Yeah sometimes we think matches like this a joke, the strongest team like Lazio played at home but could not win or maybe they think it's safe for the next round so that games like this are not important, Ludogorets Razgrad is the 4th worst team in this competition from 5 matches they lost 3 times and draw once so no one thinks they can draw against Lazio.
I think that's the beauty of football for neutral fans. If it was that easy to predict the results based on the line-up then we don't have to play the game and just run a simulation to decide the champions. I remember Liverpool also lost to Red Star in the UCL a few years ago, no one predicted they'd play badly even though their line ups are quite strong. Heck, people even predicted a Leverkusen vs Liverpool Europa League final last season right?
I guess Slot is right that these players are just human beings. They can play terribly for no reason even against a weak opponent (or can't score goals even though they dominate) and fail to capitalize on a good opportunity to secure points.