Define "Mainstream media".
Do I trust the Sun/Express/Daily Mail - barely, there is usually some element of truth buried deep beneath layers of effluent and opinion, but you need to be pretty good at working out what is opinion and hyperbole to get to it. Always check with other sources.
Do I trust the Guardian/Telegraph - Yes to a much higher degree, they're usually at least trying to be factual and do a far better job of checking things are correct for the most part (if not running a spellcheck in the Grunard), but I bare in mind that they have strong political opinions/slants.
Do I trust Sky/BBC - yes to a fairly high degree, both are required by law and regulation to be factual and at least try and give both sides of most viewpoints as part of their broadcast licence - I tend to trust the BBC a bit more than Sky mainly because the BBC tends to be a little more careful in fact checking and in less of a rush to break news..
Do I trust RT - about the same level as the Sun, their editorial policy is direct from the Kremlin which means for anything with a political bent they're on dodyg as heck ground.
Do I trus t Al'Jazeera - to a reasonable degree, again baring in mind possible leanings towards certain viewpoints.
Do I trust Fox - not at all, they're the Sun of the broadcast news (sorry Entertainment, must remember they argued that in the US, they're Entertainment, not news).
Ideally you should get your news from at least two different sources, preferably ones that have a history of reliability.
In short it varies, but I don't automatically discount "MSM" or assume that alternative sources are more accurate, non MSM news sources can be accurate, but they are highly varied from Alex Jones up, and often require a lot of fact checking.