Lack of strength is definitely his problem. He has a good burst but can't finish because he can't disengage and can't Bull-rush.
Marinelli's scheme and coaching is a little odd in that he really does not like DE to rush wide and deep into the backfield (The DWare style). He likes them to go more straight at the OT (Not actually straight at him, but more so than many pass rushers). The trade-off is that he allows them to pass rush immediately and not read-and-react to play the run. By not rushing wide and deep it keeps them more in line to defend the run despite coming off the ball as if they are pass rushing.
Having said the above, he does give them a full go occasionally on 3rd and long.
The end result of this style is that speed rushers can struggle. Marinelli's DEs need some power especially with their hands and Mayowa just doesn't have that type of strength/power.
It makes me wonder if a drug-free Randy Gregory would struggle in this scheme also. IIRC he did struggle when he played.
Mayowa is listed at 6-3, 240.
Ryan Davis is listed at 6-2, 260 (He looks bigger and probably is considering that he was played DT in the past).
All of the other DEs are 270 and over.