A bit of a broad question, but no, having a tab bar/bottom navigation for mobile and a hamburger for desktop is not inherently bad UX.
The key is to use strong design patterns.
Mobile and desktop serve a different purpose and have a different style of use.
Comfortable thumb reach and limited screen estate are simply issues that desktop designs don't have to keep in mind that strongly. Therefore, different UX is actually desired for different screen sizes. By using design patterns, you avoid having to teach your users new behaviours and expectations, regardless if your desktop and mobile designs are slightly different from one another.