This is something I've seen happen frequently in my social media feeds and I feel it's my duty to aid all letterers of webcomics, manga, and comic-style mediums. We're a global community and resources like this should be readily available and widely used for the good of all of our readers.

First of all, if you're using a mono-case font, that's great!
The choice of whether to use mixed- or mono-case typefaces for your project is something that should always cross your mind at least once. I typically find that mono cases have a more traditional, comic-book style whereas mixed-case fonts are a little bit more literary, old fashioned, and (at times) feminine.

I love mono-case fonts because you don't have to worry about whether to start a new bubble with an upper-case character or not when the previous sentence ended with ellipsis. I love that I'm essentially working with 26* letters instead of 52*. There are dozens of great, free fonts to play with, and hundreds more that creative typesetters make and sell for cheap on various websites. Choosing a typeface has never been more fun.
For free to use mono-case fonts I'm personally a fan of CCMeanwhile. Its thinness makes it nice for lettering Japanese manga which often have thin, vertical bubbles.

But if you observe the example below, you may notice that not all letters are so straight forward. This is the crossbar I.

Crossbar I is typically only used in instances of the personal pronoun "I". In all other cases, even at the start of a sentence (even as an acronym like MIB, FBI, etc), when using a mono-case font in comics you should always use a normal, non-serif "I". It's worth noting that this rule doesn't apply with mixed-case fonts.

It's my observation that a lot of creatives are unaware of the importance of the crossbar I and, to an untrained eye, it may seem like splitting hairs, but I promise you, your work will look a lot cleaner and more professional if you adhere to this simple guideline.

There are exceptions, and as time goes on this principle of lettering has shifted slightly. If you genuinely can't stand the crossbar 'I', or if you think it's the greatest thing ever and want to use it everywhere, it's not the end of the world. An accepted alternative is just to be consistent with your decision.

The times it stands out most is when an errant crossbar I starts a sentence or appears as a capital 'I' because the writer instinctively typed a capital 'I' in a mono-case font. Since no other character discriminates between lower- and upper-case it creates an awkward disconnect for the reader.

Every decision we make in communication has a value and a purpose.

Happy Lettering.