My guitar seems to be out of tune everyday.
Is it because of the bad quality of this guitar, or is this normal?
Whether that is good or bad will depend a lot on the type of guitar and where you use it.
Type of guitar:
So think about where your guitar fits in to the above and you can get a good idea whether tuning it every day is appropriate.
As an indicator, my main gigging guitar only needs very minor tuning between gigs 2 weeks apart: it is a very nice Ibanez with a locking nut, Floyd Rose and Elixir strings and it lives in a room which has a pretty constant temp between gigs.
You're correct in thinking that inexpensive guitars will go out of tune more quickly. For the most part, how much this happens depends on the quality of the guitar, how well they're set up, and how well you strung them.
I can leave some of my guitars for a week and they'll still be mostly in tune, although high humidity will often mean they're sharp. In low humidity, they'll often go flat just a touch.
But having to tune the guitar every day isn't unreasonable at all, if you're tuning no more than, say, a half-step per day. (Even that seems like a lot, actually.) If it's more than that, I'd be concerned.
In my case, I have a strat with standard bridge and I have to tune it up after every song.
Depends both on the instrument and the conditions. Surprisingly, my stable of cigar-box guitars stay in tune better than my "real" instruments.
I have to tweak my little Taylor GS Mini nearly everytime I take it out of the case, but my Cigar-box resonator will stay in tune for weeks....
Humidity, temperature, construction of the instrument... All play thier part.