No it isn’t - I weigh 60/61kg - I have a huge variation in FTP - 300W last year in the big ring on my kickr snap, 265 W in the small ring at the moment and about 250W last year for a 25m TT on my TT bike - regardless of what you take you are correct that I get my butt kicked in flat TT by guys with worse w/kg than me who have good Cd but it wouldn’t improve if I put on weight because you catch more wind…and I won our club hill climb easily 
…you need to focus on putting out more power and keeping your weight constant - that’s what I’m trying to do…especially if you want to climb.