Yes, there is clear evidence from building work, extensions and loft conversions going on to suggest that Mitcham is up and coming (and lots more people around jogging - always a sign of gentrification!). It makes sense as it's really the only affordable part of Merton left for young, public sector families. So as MitchamBelle says, buy now before you're priced out of the market.
Mitcham does have bad press and undoubtedly there are some areas that others in the borough would disdain, but it's a very diverse community which makes it vibrant and 'real', probably rather like parts of Wandsworth used to be (particularly Battersea - and look how that's changed!).
Agree with MitchamBelle that we too don't really go into Mitcham (ever!) but that's because it's effectively 'out of our way' in terms of the transport links we use, where we go out, where the DCs go to school etc...but that's possibly because we've lived in the area since it was less up and coming than it now evidently is.
The Gorringe Park Pub is great, a bit boho, community spirited and definitely chichi (but in a good way) and on the Merton side of Tooting Borders - and there, if you need the evidence, is a small green shoot of what will come and spread through this erstwhile 'unfashionable' side of the borough.
It's got good transport links, particularly up where you're considering living. Have never ventured up Western Road or Church Lane late evening, couldn't pass judgement on how safe you'd be on foot, but would imagine that at usual commuting times you'd be absolutely fine - in fact it would be just a case of observing the same safety walking home practices that you'd adopt anywhere in London, from Notting Hill to Neasden, Knightsbridge to Kenton.
Just to say that only once in all the time we've lived around here, have we experienced anything approaching 'trouble', and that was around the Tandem Centre on a summer's afternoon (so technically Colliers Wood anyway). And the incident could just have easily happened in Balham, Wimbledon or even Kingston. The young men involved were obviously high on dope and out to impress an accompanying girlfried (so all brash bravado).
And what's more there is actually some fantastic and interesting housing stock, if you look around. I'm sure the same would have been said of Balham thirty-forty years ago!
In many ways, I would say that people in Mitcham look after their own, even if you're not to the 'Mitcham manor born and bred' 
, and that's possibly where its diversity works to 'incomers' advantage!
Hope that helps?