Unlike emos which were a specific style people chose to participate in, doomers are a common western phenomenon of disappointed White youth that just happen to share a common thread. Much like the "30 year old boomer" meme, it's not an intentional style so much as an observation.
Emos "intentionally feel feels", the Doomer no longer feels. The emo's gf left him and he cries for a month… the doomer can't connect with most women, seek out broken girls with tonnes of baggage or somehow stumble onto a good relationship but intentionally sabotage it due to self-destructive tendencies. And when it's over… he feels relieved because he knows he's unlovable.
The emo is characterized by high-highs and low-lows… the doomer is characterized by disconnect and alienation. Usually a failed romantic, he differs from the goth that actively seeks apathy, the doomer may seek happiness or most likely has given up on it… however there is a reason the Bloomer meme exists, as well. Sometimes it's just a emotional/psychological blockage that can be worked by actively stewing in the misery and there's light at the end of the tunnel.
The emo is usually suburban and wishes he could "live somewhere cool like the city" whereas doomers tend to be from the city and wish they could transfer themselves to the countryside. Emos, politically were leftist… usually tepid Democrats that saw the party as "less shit than the Republicans" whereas doomers are usually eco-fascist in spirit however too pessimistic of the world to actually fight for it.
Unlike goths and emos, the doomer puts little effort into his outward appearance. The "uniform" being a black hoodie, plain knit hat or baseball cap, plain black jeans or sweatpants and plain boots or sneakers usually bought at Walmart, Amazon or whatever your country's equivalent of the IDGAF store. Alcoholism, tobacco and low level drug abuse is common.
Unlike the emo that WANTS to be emo but doesn't label himself as such, the doomer usually labels himself that way… but wishes he weren't.