I don't think she needs to be serious or sad all the time at all, I don't begrudge her joy in her life. I guess I'm just not really understanding the point to them. She may be attempting to be silly, but I just find them strange; maybe that's why they don't seem to be taking off either. She seems to be having a difficult time pulling people onto cross platforms, I notice.
It's because she's not following anyone else on Tiktok. I really doubt she has a private account there since the main one is fairly small. Tiktok in particular has so many fleeting trends and insider jokes that she needs to be very active on there to keep up. Certain audios are only meant to be used for certain situations, and the same clip posted a few weeks too late will seem out of date and cringy. There are many cancer vloggers on Tiktok as well who use it as their main platform and the quality of their content is extremely high.
I feel like she never consumes other people's content on social media for mental health reasons, which is fair enough as it's probably hard to see healthy people going about their daily life. She only follows a handful of close friends and family. However it also means she never knows what the current trends are, which are particularly relevant for Tiktok. Her own Youtube videos have the same Zoella format for over 10 years including rigid upload days which is quite dated.
Someone in the last thread mentioned covert narcissistic tendencies, which I felt was a bit extreme at the time but with the past few videos it actually makes some things fall into place. There is clearly something about her channel that feels "off" and many people can't put their finger on it plus it's enmeshed with the guilt of not wanting to think badly about someone with a serious illness. However one telltale sign is her effusive thanking and declarations of love toward her TT family. This feels disproportionately over the top but also very effective and carries classic hallmarks of love-bombing. It contributes to the sneaking feeling of manipulation that some people are sensing, but still works extremely well on the majority of viewers. Hence the copious amounts of gushing comments that support everything she says or does, which clearly give her supply. This is also why she's addicted to uploading so often regardless of whether there is sufficient content or not. She knows that whatever content is spun out for each video, her fans will still provide the supply.
The whole convoluted timelines and leaving out information (past surgeries, current results, the staging letter that one oncologist said they'd send) could just be her way of gaslighting the audience after love-bombing them to stay. She knows she can literally post or talk about anything and still receive supply so she's simply enjoying herself in this ideal space she's created. She doesn't need to give those answers because people asking for them will be chased off as haters and that in turn only generates more comments of support = more supply.
For someone who is self-professed quite introverted (you also don't see a lot of friends in her videos) she still loves attention. This is clear in attending all the appointments wearing her own merch which inevitably draws a lot of attention to them, an experience that would be quite mortifying for the average person. She also tends to react very passively aggressively when things happen that don't please her, like people badmouthing Matt or when she feels cornered for using the word cancer in every video title. You can tell she's seething with rage but trying to give an explanation that her followers would applaud her for in the comments. Last but not least, the fact she doesn't follow other accounts on social media could also be the narc tendency to be jealous of others. Granted, she IS in a position where not consuming social media is perfectly justified, but the behaviour is slightly incongruent in the context of her being extremely active with putting out content herself.