Mothma's spoiled daughter and Karn's mom are supposed to be cringey lol. The other characters having to put up with their shit while doing their day jobs is a big part of what makes this show awesome.
If anything, Cassian should have had MORE trouble piloting that craft. In the easter eggs video I posted above, apparently it's a TIE Avenger, a prototype of the TIE Interceptor that was never put into production, mainly because it was made by the same company that made the X-wings that Anakin and Obi Wan are piloting in the opening of ROTS and flies too differently than regular TIE fighters. So it's a one-off craft and Cassian goes in clueless. It was a stretch for him to blast his way out of the facility and outmaneuver trained TIE fighters in pursuit but I'll allow it.
But he shouldn't have been able to just swoop in from orbit and blast the stormtroopers like MF'ing Boba Fett in the Slave 1. He should have flown in crooked (because one of the engines got disabled from his fumbling), tried to blast the stormtroopers but missed (because he couldn't get the tracking to engage), and finally resorted to blasting the grain silos to smother the ground troops and then landing on a troop carrier, crushing it because he can't get the missiles to launch.
And the rebels on Yavin are supposed to be idiots - that's the point. IRL people do stupid shit every day and everyone in this show is a regular person, which means they're either an idiot or they have to deal with idiots on the regular, even canon bigwigs like Mothma.