I think your idea of what is best for the neighbourhood differs to mine.

As someone with asthma, and who has had to spend time in A&E with a child having an asthma attack, anything which improves air quality in Lewisham has to be a good thing. London has the poorest air in the UK and vehicles are the biggest contributor to air pollution. Trees can help to reduce air pollution. So I am sorry but if some birds do make a mess on some cars, its a small price to pay for the trees helping to mitigate the negative effects of those cars. What is most important here?
Car ownership in Lewisham is increasing every year; yet in Forest Hill at least we seem to be chopping down more trees than we are planting (that is anecdotal but its what many people have noticed and I have not heard anything to suggest otherwise).
Air pollution causes many health problems, including asthma, which have a negative effect on neighbourhoods - children missing school, additional strain on GPs, people missing work, and so on.

So no, funnily enough, I don't think more street trees is bad for the neighbourhood.

It would be nice if some of the money Lewisham collects from parking fines could be put towards more trees, but I can't see it happening somehow....