Do people tend to judge if a PhD candidate mention that upon graduation, he/she is aiming for a teaching position in polytechnics? I often get that 'why do you aim so low' or 'why do you even choose to do a PhD even'.

is this thought about career option not something to be shared amongsts your coursemates?

That is now two posts where you seem overly concerned about what other people think.
People are idiots. We all are. It really is best just to avoid this kind of converstion. It is of no consequence to anyone but yourself.
For example, I have a PhD and I have chosen to earn money delivering pizza leaflets for the moment. What other people think of that is worth precisely what I paid for their opinion.

If this makes you happy then go for it. Most of your fellow PhD students will never find a permanent academic job anyway.

There are no polytechnics in the UK anymore, as far as I know. They've all been universities since 1992.

It 's true that the newer universities (ex-polytecnic) can be less research-focused, and have less funding for research. It depends what you want from your career. If you prefer to focus on teaching, it makes sense to go where your teaching skills will be prioritised and valued, which may not be the case at some prestigious research-focused institutions!

Oh absolutely. Don't you know that the only respectable academic career is the traditional lectureship, with as little teaching as possible?

In fact, in a conversation with a professor the other day, we were talking about who was leading undergraduate workshops and she said "Not academics surely? Oh no, it's teaching staff." We may be on academic contracts, some of us permanent ones, but we will always be inferior to traditional lecturers in their eyes.

I am in a field where getting any lectureship is a big achievement these days. Some junior staff at ex-92s are truly excellent. My eyebrows would be raised at the OP's statement but largely because most people apply for any job in our sub-field regardless of the institution and it would seem a bit clueless to assume getting a job at a post-92 was a walk in the park.
I would be wary about lumping all the former polys in one group by the way, some expect research much more than others. They have diverged quite a bit as a group.