I think… that the main reason why we try to bring others down when we see them striving to become better is that we aren't there yet, and we find it hard to accept that we are below average. Seeing others, with whom we were once on a similar level of faith, trying to step ahead makes us feel threatened; sin loves company. We try to bring them down so we don't appear to be the only ones in the wrong and to feel better about ourselves.
It is quite sad that when we see people trying to do a little more than the bare minimum, we start to call them names. If, by avoiding a sin and doing more good deeds, a person becomes an "ustadh/ustadha" or a "sheikh/sheikha," what then is expected of the average Muslim?
When you see a person trying to do better and you aren't quite there yet, resist the urge to throw words of mockery. Say a prayer for them so that Allāh may instill some guidance in your heart too.
Abū Dharr (رضي الله عنه) reported that Allāh's Messenger (ﷺ) said:
“There is no believing servant who supplicates for his brother behind his back (in his absence) that the Angels do not say: ‘The same be for you too’”.
Muslim, 2732.
A simple “بارك الله فيك (may Allāh bless you)” could result in your guidance too.