Alpha characterizations really depends on you and what you find the most attractive in someone that would define an alpha
For me It's not only mark's role as the leader in dream that makes me portray him more as an alpha, but also the way nct overall see him and respect him. I see Mark as someone wth a great sense of responsability, he is very ambitious and can be very stubborn when there is something he wants (or someone since we are talking alpha/omega). With the way there is such a codependency in dream with him that has been expressed multiple times, I just cant think of him as an omega most times. There is nothing wrong with it (i have ships where i dont mind him as a bottom, but mostly i am a top mark stan) everyone has their own characterization, but for me seeing that kind of relationship makes me think of him as someone that people would follow at the end of the world. In abo terms, this translates to an alpha and his pack.
Mark is also kind and thoughtful, he seems to look after others more than himself, which this could also apply to an omega but for me an alpha that would do everything to make his omega feel well takes care of is very sexy.
I dont consider strenght a necessary trait for alphas but I guess I will put this in too cause omegas are usually looking for safety and Marklee multiple times have showed great strenght or in general a very powerful aura around himself that I feel like it draws you in and it would make every omega gag a little
He is also stupidly funny and I feel like that's a precious trait to give to an alpha like your omega is sad and going through a messy day the least an alpha can do is to make his omega's day better

Do i need to mention hes got a really big