Diniro didn't identify himself as a cop in the Diner. For all Spyder knew, it might have been another dealer trying to call smoke him out.
Blasting through the motel window was a low-risk way to attack a challenger - and set them straight for calling him out. There would be no witnesses that saw him walk into the room, no risk of getting in a tussle and leaving behind evidence, etc. Whether it was Spyder himself, or someone he paid to do it, that would be a perfectly plausible way to "send a message".
The only problem I had with it was when he then drove the truck right back to his shop and to do some business on the street corner. You can imagine the detectives interviewing the junky on the corner: "So crack-head Bob, that night Spyder came by for his pickup what was he driving? I suppose you didn't know that truck was seen shooting out a motel room where someone was killed about 10 minutes earlier would you like to testify or be charged as an accessory?"
Obviously it only works if he drives a stolen truck for the drive-by, then ditches it on his way back to his business.
It did seem un-theatrical, but that's not the same as being ridiculous. A 15 minute slug fest where guys fly through walls and crack toilets in half with their head and still get up - that's ridiculous.