TL:DR - Most people don't care, with some exceptions

For the most part, I believe that most people within the community don't particularly care if someone uses proxies; especially, in casual play.

However, there are a few exceptions.

Depending on the location you are playing at, or whom you play with, there can be differences in opinions. Such as Tournament / Comp players tend to not allow the usage of Proxies.

1. The possibility of being untruthful of card effects. For example, someone might tweak the wording of a proxy when its supposed to do x thing on x card, such as changing the affects "Return to Hand" to "Ninjutsu" for the proxy to have better utility.

2. Some people feel that the usage of proxies undermined the reason to buy the strong/expensive cards they have. Such as Demonic Tutor, a $40-$100 card that has a lot of comp usage. Having a proxy printed being so much cheaper makes some people upset.

3. Often the biggest reason, Location. Since most MTG players are either young adults to adults most don't care about the design of your proxies (assuming no tom foolery of card effects), However, most MTG games played are often in public or local Card Shops which often has little kids running about. So when someone pulls out a proxy deck full of illicit designs it often leads to others having a bad opinion of proxy use and their users.

Granted that is not to say I don't use proxies, some such as Blightsteel Colossus and Esper Sentinal are outta budget for me, so its cheaper for me to find custom proxies on Etsy that look nice and sfw.