I've been in other countries where malaria prevention is a must like Senegal or Kenya and almost all low cost accommodations include a mosquito net (sometimes broken, but in general in good condition). My doubts about that specifically for travel in India : Is it easy, in general to find low cost rooms that includes a mosquito net?
I wouldn't bet on a mosquito net being available in any low cost options.
As for the mosquitoes, it really depends on the season and the city you are travelling to. Some cities/towns just naturally don't have a lot of mosquitoes in any of the seasons.
Since one can't be sure of the guarantee of the availability of a mosquito net (and of those that are available, most are useless since just a small tiny hole in the net is sufficient to ruin your sleep. Indian mosquitoes are really tiny (and sting like hell !) compared to the average mosquitoes in US homes), allow me to list a few almost sure shot ways of sleeping mosquito free. Your options may change if a child is involved in your travel. :
I think only in extreme cases will none of these three options be available in India.
After travelling around India last year, I wouldn't bet on a mosquito net in hotels - most don't have one. Midrange hotels (still reasonably priced for Western standards) often offer aircon, so nets are theoretically not necessary - but even then we usually had at least one mosquito in the room. I don't think I would invest in a mosquito net though, consider (a) insect repellent (b) Raid plug repellent.
However, if you are only concerned about contracting malaria, then you should make sure that there is malaria in the area you will be travelling in. E.g. the UK's NHS provides this map:
The most touristy places around New Delhi, Jaipur and Agra don't have any malaria risk.
Nets are light enough and cheap enough and malaria is nasty enough that carrying your own seems a sensible option.
Taking a different approach, if you can tolerate it, a low dosage (100 mg/day) of doxycycline antibiotic is relatively effective against malaria in areas where quinine and quinine derivatives have become ineffective. Some people report various side effects from doxycycline. I have taken it at the 100 mg/day rate on several occasions with no noticeable side effects. YMMV.
If I was travelling in high Malaria risk areas in India I would probably personally carry my own net and take Doxycycline. Note that Doxycycline does not prevent you from acquiring malarial infection, but instead deals with parasites in your liver and bloodstream. Treatment needs to be started a few days before entering the infected areas and continued for some weeks after leaving them. The continued use at the end is because the effectiveness against parasites in your liver is not as good as with some other drugs but it is effective against parasites leaving your liver. Charming!. Doxycycline for Malaria Chemoprophylaxis and Treatment: Report from the CDC Expert Meeting on Malaria Chemoprophylaxis provides a good description of what Doxycycline does and how well it does (or doesn't) do it.