Trish, Dez and Austin are bad role-models, but it is worth noting that Trish and Austin are very good friends, and any mistakes they make they are extremely sorry eventually. Austin's flaw is his start, he steals Ally's song, "accidentally", convinces himself he wrote it, uploads himself singing it online and he becomes viral, and even though he apologizes, the impact of him doing so is the premise of the series.
Trish is very lazy, which isn't a big problem, she is rude, which can be overlooked, but that doesn't stop it from making her a bad character, however her constant jobs are often contrived and annoying and they do not make for a good joke.
Dez is really dumb, which isn't a bad thing, but it literally stems from him just being stupid, no reason, no possible explanation and it is so over exaggerated and sometimes he proves himself to be smart but goes back to being dumb which rids the viewer of any satisfaction that he said something worth while and not comedic.
Overall, the show is good and okay and enjoyable to a certain extent, but it hasn't got good role-models (with exception of Ally), often the over exaggerated high jinks and shenanigans can raise a few chuckles and laughs of the younger and sometimes maybe older crowds.
PS: This section is about the 'Ausally' arc as it is commonly dubbed is a major theme in the show, revolving around the titular characters getting into a relationship, at first it doesn't work and eventually they do reconcile and it works out, but it does have rather harsh consequences, but only to forward the shows plot.