It depends on what you’re wearing. Synthetic jacket? Backpack is OK. Wool coat? Sports coat? You can dress however you want, but why did you chose that way in particular man
I like the Osprey Daylite Tote because it’s a convertible backpack<->tote so you can switch. Also qualifies as personal item on low cost airlines
It has to do with the “fashion language” and what each item “is speaking”.
To me, a synthetic jacket speaks “trekking” (even though you can wear it anywhere, not necessarily trekking), a backpack is also “trekking”-ish so it matches in my head. But if you have a tailored wool sports coat with clean silhouette lines, which speaks “city”/“elegant”, and you slap a synthetic (or any for that matter) backpack on top it clashes to me. I guess it would also wear down the more delicate wool weaves from friction.
Say, I wouldn’t want to wear a backpack with something like this or this
But to me it would be more fitting with something like this or in general when trekking around somewhere.
Again, do what you want - I concede that backpacks are more comfortable. To me it just looks better with certain clothes and worse with others.